Account for day-to-day variability
Chronic kidney disease is a common condition in companion animals that follows an irreversible renal injury secondary to infections, exposure to toxic chemicals or drugs, or conditions causing decreased renal blood flow. The nature of chronic kidney disease is that it progresses over time. Therefore, monitoring of renal function is an important aspect of management.
The common indirect serum indices of renal function are serum BUN, creatinine, and SDMA (symmetric dimethylarginine assay). While the levels of these indices are affected by the degree of kidney functionality, they are also affected by day-to-day variability (i.e., biological variability and by analytical variability). Knowing the magnitude of biological and analytical variability is important for ascertaining whether an acute exacerbation in renal function has occurred when reviewing a recent laboratory result.
In a study that I performed at Massey University, New Zealand, I calculated the critical difference for serum creatinine and SDMA in dogs (https://doi.org/10.1111/jvim.15050).
I found that only a difference between two consecutive samples from the same dog that is greater than 23.7% for creatinine and greater than 47% for SDMA can confidently be regarded as representative of deterioration in renal function. A difference between two consecutive samples from the same dog that is smaller than or equal to 23.7% and 47% for creatinine and SDMA, respectively, may still represent deterioration in renal function, but could also represent the day-to-day biological and analytical variability in these indices.
Clinicians should take that information into account when interpreting these results.
By Arnon Gal, DVM, MSc, PhD, DACVIM (SAIM), DACVP (anatomic)