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?
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.
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