Calculating the Dose for the Dexamethasone Suppression Test: Actual vs. Ideal Body Weight?

I'm doing a low-dose dexamethasone suppression test (LDDST) on an 11-year-old female spayed Blue Healer cross dog. She is extremely obese (body condition score of 9/9). The dog weighs in at 59 lb, while the ideal body weight would be more like 30 lb.

Everything that I have read has mentioned dosing using the dog's actual body weight. Is this correct? Or should we be calculating our dose (0.01 mg/kg) of the dexamethasone based on ideal body weight or body surface area? In an obese patient like this, the dosing error (either way) could be as much as a 2-fold difference.

While I'm on the subject of LDDST testing, I have a second question: When calculating the dose of dexamethasone for the LDDST, I use dexamethasone sodium phosphate (5 mg/ml) for the test. I have always calculated my "low dose" as the active portion of the dexamethasone (4 mg/ml), with the other 1 mg/ml being the inactive sodium phosphate salt. In other words, I have based my calculations on the basis that dexamethasone sodium phosphate contains 75% active dexamethasone.

Recently, I was told to calculate the actual dose of dexamethasone by calculating my 5 mg/ml dexamethasone sodium phosphate as only 3 mg/ml of active dexamethasone. Which is correct for my calculations?

My Response:

First question: Yes, we have always used the actual body weight to calculate the dose for the LDDST, not the ideal body weight, metabolic weight, or body surface area (1-3). However, you make a good point about the effects of obesity, and some investigators believe that the ideal therapeutic drug doses (e.g., thyroid hormone, chemotherapeutic drugs) may correlate better with metabolic rate than with body weight (4-7).

Overall, I feels that it's better to "overdose" and see cortisol suppression than make a misdiagnosis of hyperadrenocorticism in a dog without the disease. Most dogs with Cushing's syndrome will not suppress on even a doubling of the dose used for the low-dose of dexamethasone. Remember that Cushing's is a clinical diagnosis— if you do see serum cortisol suppression but still suspect the disease, other adrenal function tests should be done. As you know, this is certainly not an exact science!

Second question:  You are correct —dexamethasone sodium phosphate contains 75% active dexamethasone.

See this blog post (Why precise dose calculation is critical for low dose dexamethasone suppression testing) for more clarification about how I calculate the dexamethasone dose for the LDDST.

References:

1. Meijer JC, de Bruijne JJ, Rijnberk A, et al. Biochemical characterization of pituitary-dependent hyperadrenocorticism in the dog. J Endocrinol 1978;77:111-118. 
2. Feldman EC. Comparison of ACTH response and dexamethasone suppression as screening tests in canine hyperadrenocorticism. J Am Vet Med Assoc 1983;182:506-510. 
3. Peterson ME. Hyperadrenocorticism. Vet Clin North Am Small Anim Pract 1984;14:731-749. 
4. Price GS, Frazier DL. Use of body surface area (BSA)-based dosages to calculate chemotherapeutic drug dose in dogs: I. Potential problems with current BSA formulae. J Vet Intern Med 1998;12:267-271. 
5. Henness AM, Theilen GH, Madewell BR, et al. Use of drugs based on square meters of body surface area. J Am Vet Med Assoc 1977;171:1076-1078. 
6. Chastain CB. Canine hypothyroidism. J Am Vet Med Assoc 1982;181:349-353. 
7. Evinger JV, Nelson RW. The clinical pharmacology of thyroid hormones in the dog. J Am Vet Med Assoc 1984;185:314-316. 

Why Precise Dose Calculation is Critical for Low Dose Dexamethasone Suppression Testing

I just read your latest blog post on the low dose dexamethasone suppression test (LDDST) and have a couple of questions. I think many veterinarians are aware that the active dexamethasone portion of dexamethasone sodium phosphate (Dex SP) is less than what the labeled concentration states. However, I also know that some colleagues do not take the "active" dose of dexamethasone vs the total Dex SP into account. when calculating the dose administered for LDDST testing.

So, from your experience, if a veterinarian fails to take into account the fact that 1-ml of Dex SP (4 mg/ml) is really only delivering 3 mg of active dexamethasone, will the actual test results be effected or altered to a significant degree? Would it really make a clinical difference?

And you mentioned diluting the Dex SP. How do you do the dilution? Do I have to make a fresh dexamethasone dilution for every LDDST I do or can I store the diluted dexamethasone? If so, how long would this dilution be stable?

My Response:

Let me address your second question about diluting the Dex SP first, because I believe that dilution is key to giving an accurate dose. This is true in all dogs, but is mandatory in small breed dogs.

How to dilute and store dexamethasone sodium phosphate preparation for the LDDST
I routinely dilute the dexamethasone sodium phosphate (Dex SP) used for the LDDST. This is my protocol for diluting the steroid, but actually it probably can be diluted more or less as needed based upon the size of the dog.

• Draw up 1-ml of Dex SP (4 mg/ml), containing 3 mg (3,000 μg) of active dexamethasone,
• Add this 1-ml volume of Dex Sp into a sterile glass vial.
• Add 5-ml of bacteriostatic 0.9% saline to the sterile glass vial containing the 1-ml of Dex SP. 
• The concentration of the diluted Dex SP salt in the glass vial is now 0.67 mg/ml (667 μg/ml). 
• More importantly, the concentration of the diluted "active" dexamethasone is now 0.5 mg/ml (500 μg/ml).
• Store the diluted Dex SP for up to 1 month in the refrigerator (1). It's likely that this diluted Dex SP is stable for much longer than 1 month, but we just don't know how long.
• Remember to protect the diluted Dex SP from light (i.e., keep in dark refrigerator).

Diluting the Dex SP is recommended to ensure administration of the correct dose

Calculating the correct dexamethasone dose for the LDDST 
Now let me address your question: does it really matter if we calculate the dose for the LDDST based on the weight of the Dex SP salt or the actual dexamethasone contained with the salt? Again, as I discussed in my last blog post, the Dex SP label states a concentration of 4 mg/ml for the dexamethasone salt, but that is equivalent to only 3 mg/ml of active dexamethasone (2).

For an example, let's take a dog weighting 20 kg that we need to test for Cushing's syndrome with the LDDST. Let's do the dose calculations both ways — based on the weight of the dexamethasone salt and then based on the active dexamethasone:

• Remember that the dose of the LDDST test is 0.01 mg/kg (or 10 μg/kg); I find working with micrograms easier since it avoids all the tiny numbers and decimal points!
• So first, take 20 kg times 0.01 mg/kg (or 10 μg/kg) = 0.2 mg or 200 μg (the dose to inject).
• If we use the 3 mg/ml of active dexamethasone for the calculation, that means we would administer 200 μg of active dexamethasone to the patient.
• In contrast, if we use the 4 mg/ml of Dex SP salt for the calculation, that means we would administer 200 μg of Dex SP salt, but only 150 μg of active dexamethasone! This calculates out to an administered dose that contains 25% less active dexamethasone than what we should be giving.

So could it really make a difference if we calculated the dose based on the weight of the dexamethasone salt or based on the active dexamethasone contained in the Dex SP? Of course it could, and that's why it is important to carefully calculate and draw up an accurate Dex SP dose to administer when doing a LDDST.

Bottom Line: 

If we miscalculate the dose for the LDDST using the concentration of the total Dex SP salt rather than the concentration of active dexamethasone, we will administer a dose that is 25% less than is needed to ensure complete suppression of the hypothalamic-pituitary-adrenal axis (3-5). In other words, by giving a lower dexamethasone dose, it would certainly be possible to see a normal dog fail to completely suppress the serum cortisol concentrations or show "escape" from suppression at 8 hours.

In other words, if we make the calculation incorrectly, this may lead to false positive test results for Cushing's disease. And that's the last thing we want to do.

References:

1. Plumb DC. Plumb's Veterinary Drug Handbook. Seventh Edition. Wiley-Blackwell; 2011.
2. Lugo RA, Nahata MC. Stability of diluted dexamethasone sodium phosphate injection at two temperatures. The Annals of Pharmacotherapy 1994;28:1018-1019.
3. Melián C, M. Pérez-Alenza, D, Peterson ME. Hyperadrenocorticism in dogs, In: Ettinger SJ (ed): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Seventh Edition). Philadelphia, Saunders Elsevier, 2010;1816-1840.
4. Peterson ME. Diagnosis of hyperadrenocorticism in dogs. Clinical Techniques in Small Animal Practice 2007;22:2-11.
5. Herrtage ME, Ramsey IK. Canine hyperadrenocorticism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:167-189.

Helpful Tips to Improve the Accuracy of the Low Dose Dexamethasone Suppression Test

Of the 3 major screening tests for Cushing's syndrome, the low-dose dexamethasone suppression test (LDDST) is considered by many to be the test of choice for the diagnosis of hyperadrenocorticism in dogs (1-3).

In this post, I want to address some of the common mistakes I see veterinarians make when performing this test. Addressing a few small but important issues in dosing and timing of sampling can greatly improve the diagnostic accuracy of this test.

Advantages of the LDDST as a diagnostic test
Compared with the ACTH stimulation test, the LDDST is much more sensitive in confirming Cushing's syndrome in dogs. The sensitivity of the LDDST is excellent, approximately 90% to 95% in dogs with pituitary-dependent hyperadrenocorticism (PDH) and virtually 100% in dogs with a cortisol-secreting adrenal tumor (1-3).

Thus, this test will fail to confirm hyperadrenocorticism in only about 5% of dogs with the disease. That's not too bad.

Disadvantages of the LDDST as a diagnostic test
In contrast to the ACTH stimulation test, the LDDST is not helpful in the detection of iatrogenic hyperadrenocorticism. The test is also affected by more variables than the ACTH stimulation test, takes 8 hours to complete, and does not provide pre-treatment information that may be used in monitoring the effects of mitotane or trilostane therapy (1-3). For more information about when to use the ACTH stimulation test, see my previous post entitled, Diagnosing Canine Cushing's Disease: Should the ACTH Stimulation Test Ever Be Used?

The specificity of the LDDST can be low (40% to 50%), especially when measured in a population of sick dogs (4).  Because of the low specificity of this test, diagnosis of Cushing's syndrome should never be based on results of an LDDST alone, especially in a dog with nonadrenal disease. It is best to delay testing for hyperadrenocorticism until the dog has recovered from the concurrent illness (4).

Published protocols for the LDDST
Two similar protocols have been described for the LDDST in dogs, both of which yield similar patterns of cortisol suppression (1-3,5). One of these protocols used the veterinary-labeled preparation, dexamethasone in polyethylene glycol (Azium® Solution; 2 mg/ml; Schering-Plough).  Unfortunately, after merger with Merck in 2009, this Azium preparation has been discontinued and is no longer available (6).

The other test protocol for the LDDST —which I currently recommend— uses dexamethasone sodium phosphate (4 mg/ml) as the form of glucocorticoid administered.

Test Protocol & Sampling times
The protocol and steps that I take when performing a LDDST on a dog suspected of suffering from hyperadrenocorticism is as follows:

1. Hospitalize the dog at least 1 hour before the start of test
2. Carefully measure dog's body weight
3. Calculate the dog's dexamethasone sodium phosphate dose to be administered (0.010 mg/kg)
4. Collect a basal sample for serum or plasma cortisol concentration
5. Inject the dexamethasone, IV or IM
6. Collect additional serum or plasma cortisol samples at 4 and 8 hours

Calculating the dose: Remember that only 75% is active dexamethasone
Again, with this LDDST protocol, the dose of dexamethasone to inject is 0.010 mg/kg, IV or IM (1-3,5). A common mistake made by many veterinarians is to miscalculate the dose of dexamethasone to inject for the test.  When using dexamethasone sodium phosphate, one must remember that the label states a concentration of 4 mg/ml of the dexamethasone salt, but that is equivalent to only 3 mg/ml of active dexamethasone (7).

In other words, if you use dexamethasone sodium phosphate, we must use 3 mg/ml for the calculation of how much to inject. Even though it says 4 mg/ml on the label, only 3 mg/ml of it is actually dexamethasone (7).

Best to dilute in dexamethasone in small breed dogs
In smaller breed dogs, it is very important to dilute the dexamethasone (with saline solution) in order to administer an accurate dosage to the dog. Otherwise, too-much or too-little dexamethasone might be administered, which could result in false-negative or false-positive test results.

Stress and other factors to avoid when performing the LDDST
It is important to remember that "stress"activates the hypothalamic-pituitary-adrenal axis, leading to increased secretion of both ACTH and cortisol. In some normal dogs, the stress response may override the suppressive effects of the LDDST to produce a false-positive test result.

Each dog's threshold to stress is different, and it is sometimes difficult to judge the severity of stress in some dogs.  To minimize the effect of stress on this test, remember the following guidelines:

• Allow the dog at least 1 hour to relax and acclimate to the hospital environment before collecting any blood or starting the test.
• Avoid performing other diagnostic procedures on the day of the test, such as radiographs or an abdominal ultrasound. If one must do one of these procedures on the same day that the LDDST is run, the radiography or ultrasonography should be performed a minimum of 2 hours before the start of the LDDST to allow the cortisol concentrations to return to baseline values (8).
• Even more importantly, anesthesia for even minor procedures (e.g., dental, biopsies) must not be done on the day of the LDDST (9-11).
• In dogs that become highly stressed while in the hospital, tranquilizers and sedatives can not be administered before or during the LDDST. Any of the commonly used tranquilizers may skew the cortisol results and make the LDDST invalid.
• In highly stressed or agitated dogs, one option is to have the owner stay with the hospitalized dog in a quiet area of the hospital during the testing day. Alternatively, a house call veterinarian can perform the LDDST on the dog in the home environment.  If neither of those options are feasible, the owner can take the dog out of the hospital after one collects the basal cortisol sample and injects the dexamethasone, then returns at 4 hours and again at 8 hours for the post-dexamethasone cortisol samples.

Interpreting the results of the LDDST
If the LDDST fails to adequately suppress circulating cortisol concentrations in a dog with compatible clinical signs, this is consistent with a diagnosis of Cushing's syndrome (see Figure below).  Although each laboratory should establish their own diagnostic "cut-off" limits, most labs have the suppression set at 1.4-1.5 μg/dl (40 nmol/L).

Although basal and 8-hour post-dexamethasone samples are most important for test interpretation, one or more samples taken at intermediate times (e.g., 4 hours) during the test period may also be very helpful. Approximately 30% of dogs with PDH exhibit serum cortisol suppression at 4 hours (<1.4 μg/dl or <40 nmol/L), with a rise in cortisol concentration 8 hours after dexamethasone administration. This escape from suppression is diagnostic for PDH, and further tests to determine the cause of hyperadrenocorticism are not necessary (1-3,5).

Figure 3: Patterns of serum cortisol suppression seen with the LDDST in normal dogs and dogs with hyperadrenocorticism.  
PDH = pituitary-dependent hyperadrenocorticisim
ADH = adrenal-dependent hyperadrenocorticism (i.e., adrenal tumor).
Figure from reference (3).

Failure to show suppression (serum cortisol remaining above 1.4 μg/dl with less than 50% of basal cortisol concentration at both 4 and 8 hours) is diagnostic for Cushing's syndrome. However, these results do not aid in determining the cause of the disorder. Further tests, such as use of abdominal ultrasound, high-dose dexamethasone suppression testing, or determination of plasma ACTH concentration, must be used to determine the cause of the Cushing's syndrome in those cases (1-3).

References:

1. Melián C, M. Pérez-Alenza, D, Peterson ME. Hyperadrenocorticism in dogs, In: Ettinger SJ (ed): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Seventh Edition). Philadelphia, Saunders Elsevier, 2010;1816-1840.
2. Peterson ME. Diagnosis of hyperadrenocorticism in dogs. Clinical Techiques in Small Animal Practice 2007;22:2-11.
3. Herrtage ME, Ramsey IK. Canine hyperadrenocorticism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:167-189.
4. Kaplan AJ, Peterson ME, Kemppainen RJ. Effects of disease on the results of diagnostic tests for use in detecting hyperadrenocorticism in dogs. Journal of the American Veterinary Medical Association 1995; 207:444-451. 
5. Mack RE, Feldman EC. Comparison of two low-dose dexamethasone suppression protocols as screening and discrimination tests in dogs with hyperadrenocorticism. Journal of the American Veterinary Medical Association 1990;15;197:1603-1606.
6. Merck/Schering-Plough Merger. Merckpr.com website. 
7. Plumb DC. Plumb's Veterinary Drug Handbook. Seventh Edition. Wiley-Blackwell; 2011.
8. May ER, Frank LA, Hnilica KA, et al. Effects of a mock ultrasonographic procedure on cortisol concentrations during low-dose dexamethasone suppression testing in clinically normal adult dogs. American Journal of Veterinary Research 2004;65:267-270. 
9. Fox SM, Mellor DJ, Firth EC, et al. Changes in plasma cortisol concentrations before, during and after analgesia, anaesthesia and anaesthesia plus ovariohysterectomy in bitches. Research in Veterinary Science 1994;57:110-118. 
10. Church DB, Nicholson AI, Ilkiw JE, et al. Effect of non-adrenal illness, anaesthesia and surgery on plasma cortisol concentrations in dogs. Research in Veterinary Science 1994;56:129-131. 
11. Fox SM, Mellor DJ, Lawoko CR, et al. Changes in plasma cortisol concentrations in bitches in response to different combinations of halothane and butorphanol, with or without ovariohysterectomy. Research in Veterinary Science 1998;65:125-133. 

Diagnosing Canine Cushing's Disease: Should the ACTH Stimulation Test Ever Be Used?

How does this test work? What are we really evaluating with the ACTH stimulation test?

First of all, the basis for this test is that dogs with pituitary-dependent hyperadrenocorticism (PDH) or functional, cortisol-secreting adrenal tumors (FAT), because of their increased adrenocortical mass or volume, have the capacity to secrete excessive amounts of cortisol.

This is an indirect test that gauges the degree of adrenocortical thickness. Unlike the low-dose dexamethasone suppression test, the ACTH stimulation test doesn’t look at the entire hypothalamic-pituitary-adrenal (HPA) axis or the effect of glucocorticoid-negative feedback on the HPA axis.

In normal dogs, administration of ACTH produces a rise in serum cortisol to values usually >10 μg/dl (>300 nmol/L). In contrast, dogs with Cushing’s syndrome (because of the increased thickness of the adrenal cortex) tend to have an exaggerated cortisol response to ACTH administration, with post-ACTH serum cortisol rising to concentrations >20 μg/dl (>600 nmol/L). Dogs with iatrogenic Cushing's have a blunted cortisol response to ACTH (see figure).

But what’s the percentage of dogs with Cushing’s disease that are actually diagnosed with this test? What’s the test sensitivity?

About half of dogs with cortisol-secreting adrenocortical tumors and about 80% of dogs with pituitary-dependent hyperadrenocorticism (PDH) show an exaggerated cortisol response to the ACTH stimulation test.

In clinical practice and reality, however, the sensitivity is actually worse than those statistics reveal. Although it is clear that over 20% of dogs with Cushing's syndrome have post-ACTH cortisol test results within the reference range (< 20 μg/dl for my laboratory), an additional 20-30% of dogs with Cushing's have test results described as "borderline" (serum cortisol concentrations >16 μg/dl but <24 μg/dl).

In effect, that means that only 50-60% of dogs with Cushing's disease will have serum cortisol responses that are clearly abnormal (> 25 μg/dl). In other word, this test is a poorly sensitive diagnostic test for Cushing’s syndrome. In most dogs, it would be more cost effective to turn to another test, such as the low-dose dexamethasone suppression test, as the first-line screening test.

Other disadvantages to using the ACTH stimulation test:

In addition to the ACTH stimulation test’s mediocre test sensitivity, there are 2 other reasons for choosing one of the other diagnostic tests over the ACTH stimulation test, including the following.

1. First, of all, the cost of the preferred ACTH (cosyntropin; Cortrosyn) is high. I’ll discuss more about why you need to use cosyntropin in my next post (I’ll also offer my suggestions on how to extend the shelf-live of Cortrosyn to make it more cost effective).
2. Secondly, this test cannot determine the underlying cause of Cushing's syndrome. In other words, a positive ACTH stimulation test doesn't tell you whether the dog has PDH or FAT, so additional testing will always be needed to determine that. It is a true screening test, and no features of the ACTH stimulation test result allow discrimination between PDH and FAT.

Are there any clinical situations where we should use the ACTH stimulation test?

Yes, there are definitely 3 situations in which ACTH stimulation testing would be indicated as a diagnostic or monitoring test for adrenal disease in dogs. The major indications for using the ACTH stimulation include the following:

1. Best test to diagnose iatrogenic hyperadrenocorticism in dogs. If the dog has clinical and laboratory features consistent with Cushing’s syndrome (e.g., polyuria, polydipsia, polyphagia, pot-belly, truncal hair loss, high serum alkaline phosphatase) but has a recent history of glucocorticoid use, this is the test of choice. If the cortisol response to ACTH stimulation is low-normal or blunted in this dog, the diagnosis would be iatrogenic Cushing’s, rather than the naturally occurring disease. None of the other pituitary-adrenal function tests can make this differentiation.
2. Best test to use for monitoring and adjusting mitotane or trilostane therapy. Again, this is the only pituitary-adrenal function test that can be used in this situation; none of the other pituitary-adrenal function are useful in monitoring drug treatment.
3. “Gold Standard” test to diagnose spontaneous hypoadrenocorticism (Addison’s disease). This is true whether the dog has primary or secondary hypoadrenocorticism or typical or atypical disease; the ACTH stimulation test is the only test to use for this diagnosis. As you can see in the figure below (an Ad from the company that manufactures Cortrosyn), the main indication for ACTH as a diagnostic test is adrenocortical insufficiency, not Cushing's syndrome.

In general, there is a general rule for endocrine disease that is useful to remember when approaching endocrine hyperfunction vs. hypofunction:

In almost all cases, it is better to use stimulation tests to diagnose endocrine hypofunction, whereas suppression tests are best used for endocrine hyperfunction.

So with the ACTH stimulation, the 3 situations (listed above) where this test really has advantages over the other screening tests all include situations where the adrenal glands have been suppressed (iatrogenic steroid use, trilostane, mitotane) or destroyed (Addision’s disease)!

Q & A: Does this Dog have Cushing's Syndrome?

I recently ran a low-dose dexamethasone suppression test on a 11-year-old male neutered Maltese with clinical features of muscle loss, thinning coat, and a pot-bellied appearance.  His serum chemistry panel is completely normal except an alkaline phosphatase activity in the thousands.  Below are the results low-dose dexamethasone suppression test, but I am confused as to what to make of them.

• Basal cortisol:  5.1 μg/dl (reference range = 1-4 μg/dl)
• 5-hr post cortisol:  <0.7 μg/dl (reference range = <1.5 μg/dl)
• 8-hr post cortisol:  1.9 μg/dl (reference range = <1.5 μg/dl)

Thanks in advance.


My Response:

The interpretation would be as follows: Slightly high basal cortisol, with complete suppression of cortisol at 5 hours, followed by escape from cortisol suppression at 8 hours.

The lack of suppression at the 8-hour testing period is diagnostic for hyperadrenocorticism. The fact that the dog suppressed so well at 4 hours tells us that this dog's Cushing's disease cannot be caused by an adrenal tumor (ie, these dogs would not suppress cortisol with either the low- or high-dose dexamethasone suppression test).

The pattern of cortisol suppression at 3-5 hours followed by escape from cortisol suppression at 8 hours, as shown here in this dog, is always diagnostic for pituitary-dependent hyperadrenocorticism.

Q & A: Cushing's in dogs...If it looks like a duck and quacks like a...

I saw a 4-year-old, MC Maltese today for a second opinion consult. The dog has severe polyuria and polydispia (PU/PD), and has also developed progressive lethargy, weight gain, extreme panting, and polyphagia over the last few months. The dog also has a marked pot bellied appearance, and has an large liver on abdominal palpation.

Results of my screening lab work is unremarkable except for a high serum alkaline phosphatase (805 U/L; normal < 100 U/L).  The urine specific gravity is dilute at 1.005.

Results of abdominal ultrasound show an enlarged liver with a diffuse increase in echogenicity. The left adrenal gland is mildly plump at 0.6 cm in diameter and 1.45 cm in length. The right adrenal gland was normal in size.



I did a low-dose dexamethasone suppression test with these results:  Pre 8.6 μg/dl;  4-hr Post 0.4 μg/dl; and 8-hr Post 1.4 μg/dl. The owner was told by other veterinarians that these results were not diagnostic for Cushing's syndrome, I guess, because of the 8 hr Post of 1.4 μg/dl.



This dog definitely looks Cushingoid. Clients refused to do a University of Tennessee adrenal panel or a urine creatinine ratio because of finances.  I am very inclined to treat with trilostane.   What do you think?

My Response:

This dog certainly sounds like he has Cushing's syndrome to me, too.  The results of the low-dose dexamethasone suppression test are very suggestive of pituitary-dependent Cushing's disease, with complete suppression at 4 hours and some escape at the 8-hour test period. I understand that the laboratory may have a diagnostic "cut off" value that is slightly higher than 1.4 μg/dl, but this test is very suggestive given the dog's signalment and clinical features.



I do agree with the owner that the University of Tennessee panel is expensive and is probably a waste of money for this dog. Everything so far is pointing to "typical" Cushing's disease so we need to use the time-honored tests for Cushing's first. In many dogs, we have to do more than one of the screening tests (sometimes more than once over to time) to confirm this disease, and you have only done one screening test on one occasion. So which should you do next, the ACTH stimulation test or measure urinary cortisol:creatine ratios (UCCRs)?



I would not bother with an ACTH stimulation test because it has the lowest test sensitivity (only 60-80%). This is another reason not to do the University of Tennessee adrenal panel. In addition, the ACTH that must be injected is expensive.



I would have the owner collect 3 urine samples at home and bring them to your hospital for submission for UCCRs. If money is an issue, you can take a small aliquot (1.0 ml) from each of the 3-day's urine samples the owner collected, and submit the pooled urine for a single UCCR result. This will then give you a 3-day average of urine cortisol secretion. If this is positive, then I'd treat with either trilostane or mitotane.



What if the owner refuses to do more tests? Then I certainly wouldn't start treatment at all. If the owner does not have the money to do a single UCCR, then how can they afford the medication or monitoring costs??  Cushing's is a very serious disease and, unfortunately, we do not have an inexpensive means of treating this disease.

 

It's time to have a heart to heart talk with the owner about this disease and what they really can afford.

Diagnosing Cushing's Syndrome in Dogs: When Should I Test? What's the Best Test to Use?

How do we best diagnose hyperadrenocorticism in dogs? Which is the best diagnostic test?  A presumptive diagnosis of should be made from clinical signs, physical examination, routine laboratory tests, and diagnostic imaging findings, but the diagnosis must be confirmed by use of cortisol hormone assays and pituitary-adrenal function tests. For screening tests designed to diagnose Cushing's syndrome, we have three choices:

• ACTH stimulation test
  
• Low-dose dexamethasone suppression test (LDDST)
  
• Urinary cortisol:creatinine ratio (UCCR)

Which screening test is best?

Unfortunately, none of the diagnostic tests used in dogs with suspected Cushing's syndrome are totally reliable, and both false-positive and false-negative results are common. Because there are inherent problems with these diagnostic tests, the veterinarian is frequently challenged when attempting to properly interpret the dog’s tests results.

It is important to remember, however, that the predictive value of a positive screening test result for Cushing's increases in direct proportion to the number and severity of clinical signs and biochemical changes that develop in this disease. Therefore, one must always remember the importance of the dog’s signalment, history, and physical examination findings when interpreting the diagnostic test results of all pituitary-adrenal function tests.

Who should be tested for hyperadrenocorticism?

Testing for hyperadrenocorticism in a dog should be done because they have one or more clinical signs of the disease. Typically the disease is insidious and slowly-progressive, so most dogs have had clinical signs, such as abdominal enlargement, panting, muscle weakness, thin skin, lethargy, polyphagia, polyuria and polydipsia (PU/PD) for months to even years before the owners recognize a problem and seek veterinary help.

Who should NOT be tested for hyperadrenocorticism?

Testing for Cushing's syndrome is not recommended if the only abnormality is an increased serum alkaline phosphatase (SAP) activity on a serum chemistry panel, and the dog is otherwise apparently healthy. It is difficult enough to interpret endocrine tests in dogs with clinical signs of the disease; if they have no clinical signs, all of the endocrine tests may be difficult to interpret because of false-positive and false-negative results. The first step in workup in these dogs may include an abdominal ultrasound or bile acid testing.

One should not screen dogs for hyperadrenocorticism when the dog is sick with clinical signs that would not be related to Cushing's syndrome (e.g., vomiting, anorexia, weight loss). Many non-specific illnesses and other systemic diseases will produce false-positive results with the endocrine tests. Remember, hyperadrenocorticism is only slowly progressive, so hyperadrenocorticism is never an emergency diagnosis.

Before diagnostic testing is performed, it is therefore always good to ask oneself: if the test results would indicate hyperadrenocorticism, would I then feel confident to start treating with mitotane (Lysodren®) or trilostane (Vetoryl®) given the clinical picture of the patient? Would treatment help the dog's clinically signs? If either answer is "no," then it is probably best not to screen for hyperadrenocorticism in the first place.

My dog has clinical signs of hyperadrenocorticism and now other nonrelated illness − now can I test?

If the dog has any or all of the clinical signs of hyperadrenocorticism, yes, you should proceed with testing of the pituitary-adrenal axis. However, it is necessary first to ensure that the dog is not being exposed to exogenous glucocorticoids, including topical glucocorticoids on the eyes, ears or skin. First check with the owner to ensure that they are not applying topical glucocorticoids to their own skin − sometimes, dogs may ingest glucocorticoids by licking steroid-containing cream off of the owner's skin. If there is any doubt, ask the owner to bring in all and any medications that dog is receiving (including over the counter preparations) and anything that the owner or owner's family might be using in order to verify that no glucocorticoids are in any of these medications.

Next, obtain a routine database (CBC, serum chemistry analysis, and complete urinalysis) before any endocrine testing is undertaken. Finding the expected clinical pathology changes in a dog with suspected Cushing's syndrome helps confirm what the initial history and physical exam suggested. Typical abnormalities in dogs with hyperadrenocorticism include high values for serum alkaline phosphatase, alanine aminotransferase, low to low-normal serum urea nitrogen, and dilute urine specific gravity.  On the CBC, we may see a  'stress' leukogram (elevated mature neutrophils and monocytes, decreased lymphocytes and eosinophils), and a hematocrit and RBC count at the high end of normal.

What about now? Now should I use one of the screening tests?

If all the analyses point to hyperadrenocorticism at this stage, test away! In an upcoming blog, I'll be discussing the pros and cons of the ACTH stimulation test for Cushing's syndrome.