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.