Evaluation of hemoglobin interference thresholds for chemical urinalysis panels

Chemical urinalysis provides rapid analysis of multiple tests simultaneously including glucose, ketones, leukocyte esterase, nitrite, bilirubin, urobilinogen, pH, specific gravity, protein and hemoglobin. While highly coloured urines may impact readability of urine dipsticks there is no specific data on the effects of visible blood for dipstick analysis. We aimed to evaluate the effect of urinary hemoglobin on 10SG multistix and establish a process for handling visibly bloody urines received in our laboratories.

In vitro bloody urine pools were made using residual RBCs or leukoreduced RBCs and tested for interference patterns among tests included on the 10SG multistix. We further examined the effect of centrifugation and lysis on bloody urine samples both in vitro and among visibly bloody patient urine specimens.

The presence of blood in urine samples caused profound interference for leukocyte esterase, nitrite, protein, glucose, ketones, and urobilinogen. Urine protein was most sensitive to hemoglobin interference at >0.2 g Hgb/L; and clinically significant interference was observed at >1 g Hgb/L with 10SG dipsticks. Bloody patient urines demonstrated similar interference patterns observed in vitro, although some clinical samples were positive for bilirubin. This phenomenon was replicated in vitro after RBC lysis. Centrifugation was not a viable strategy to eliminate interference due to blood.

Visibly bloody urines are wholly unreliable for chemical urinalysis with 10SG multistix when hemoglobin is >1 g/L. An optimal process was established to withhold results of visibly bloody urines when dipstick hemoglobin is 3+ and protein is >1+ using 10SG multistix.