Evaluation of pre-analytical factors impacting urine test strip and chemistry results

Objectives: Careful consideration of the pre-analytical process for urine examination is essential to avoid errors and support accurate results and decision-making. Our objective was to assess the impact of various pre-analytical factors on urine test strip and quantitative chemistry results, including stability, tube type, fill volume, and centrifugation.

Methods: Residual random urine specimens were identified. Stability of 10 urine test strips and 13 quantitative chemistry parameters were assessed at eight time points (2, 4, 6, 8, 24, 48, 72, and 96 h) at room temperature (RT) and 2-8 °C (n=10-20 samples). The effect of additional pre-analytical variables was assessed, including using preservative tubes for urine chemistry as well as preservative tube underfilling and centrifugation on urine test strip results (n=10 samples).

Results: Seven of the ten urine tests strips evaluated met the minimal agreement criteria for stability (Cohen's kappa >0.70) across all conditions. A Cohen's kappa value of <0.70 was observed for pH (48 h), glucose (72 h), and protein (96 h) at RT. All 13 urine chemistry analytes remained stable at defined time points and conditions. Underfilling preservative tubes for urine test strips and centrifugation demonstrated no significant effect. The impact of using preservative tubes for urine chemistry was negligible with the exception of sodium and osmolality.

Conclusions: These findings highlight the pre-analytical factors that impact urine specimen evaluation and may be useful in informing clinical laboratory practices. Acceptable stability window for urine test strips should be considered in the context of the proportion of pathological samples evaluated.