Are urea and creatinine values reliable indicators of azotaemia in canine babesiosis?

Serum urea and creatinine are extensively used as parameters to screen for azotaemia. Their reciprocal plots roughly correlate with glomerular filtration rate (GFR). They are, however, subject to influence by non-renal factors and to increase their specificity they are often tested concurrently. In renal disease they are expected to behave similarly, with both parameters increasing as GFR decreases. Haemolysis, as it occurs in canine babesiosis, may cause non-renal elevations in serum urea, possibly due to ammonia loading. Furthermore, haemolysis with its related elevations in serum bilirubin and serum haemoglobin, may negatively bias the measurement of serum creatinine due to interference of these substances with the chemical analysis of serum creatinine. This negative bias occurs when the alkaline picrate method, or when direct enzymatic methods based on the measurement of hydrogen peroxide, are used. In order to investigate the significance of these perturbations in canine babesiosis, paired values of serum urea and serum creatinine from Babesia canis-negative, non-haemolysis dogs (Group 1), were used to establish a relationship between urea and creatinine over a range of azotaemia by linear regression analysis. This relationship was then used to predict serum creatinine values from actual serum urea values in B. canis-positive dogs (Group 2). The mean of the predicted serum creatinine values for Group 2 (237.03 micromol/l) was then compared with the mean of the actual serum creatinine values for Group 2 (131.31 micromol/l). For Group 2, the mean actual serum creatinine demonstrated a significant negative bias relative to the mean predicted creatinine value. There was also a higher correlation between serum urea and serum creatinine in Group 1 than in Group 2. These findings may have been caused by either nonrenal elevations of serum urea values or by interference with the measurement of serum creatinine. Therefore, although it is possible that some Group 2 dogs with B. canis with high serum urea and normal, low, or zero values for serum creatinine were not azotaemic, it is also possible that other Group 2 dogs with these biochemical findings did in fact have azotaemia. This study concluded that urea and creatinine do not behave in a similar and predictable manner over a range of azotaemia in canine babesiosis and are therefore not ideally suited for the detection of renal disease in this clinical setting.