It is very common for clinicians to call the laboratory asking how long they should wait before sampling a skin problem that has already been treated empirically with glucocorticoids.
If one consults standard texts on veterinary dermatology, these generally contain recommendations to wait because “anti-inflammatory agents can dramatically affect the histologic appearance of many dermatoses”.1,2 In particular, the pattern of cell infiltration is often important in diagnosing immune-mediated diseases.
The general recommendation is stop oral glucocorticoid treatment 2-3 weeks before biopsy1,2; 30 days would be ideal. The wait period after repository glucocorticoids should be 6-8 weeks1. Note: There does not seem to be any need to wait with Apoquel and Cytopoint.
It might seem that this is too long to wait in an animal that is very pruritic or has severe skin lesions, compromising its welfare, or pragmatically this may not be convenient for, or understood by the owner who wants an answer now.
Personally, I think that there may be grounds for taking a biopsy sooner than two weeks, especially if the lesions quickly return to their pre-treatment clinical presentation. There are some diseases in which glucocorticoid treatment will not obscure the underlying problem (e.g. actinic furunculosis, epitheliotropic lymphoma) and so waiting might turn out to have been unnecessary. However, there are a number of important factors to consider first.
Is a biopsy even indicated? Could the patient have a sinister problem that needs to be diagnosed by biopsy, or is the presentation more likely to be allergic skin disease, which has non-specific histological features?
The next consideration is that secondary infections of bacteria and yeast should be controlled before biopsy or at least under treatment at the time of biopsy, because those are pruritic and also “may obscure the histopathologic features of concurrent dermatoses.”1 Furthermore, epithelial damage by infection can mimic immune-mediated diseases histologically (e.g. pemphigus foliaceus).
Therefore, overall we do not think a blanket recommendation is possible and recommend a case by case decision on if and when to biopsy, integrating careful consideration of case presentation, signalment, signs and management so far. Veterinarians are welcome to call a pathologist and discuss this as required.
1. Muller and Kirk’s Small Animal Dermatology, 7th edn. Elsevier Saunders, 2013.
2. Equine Dermatology, 2nd edn. Elsevier Saunders, 2011.