Stability of common analytes in urine refrigerated for 24 h before automated analysis by test strips

Central outpatient laboratories might find processing large numbers of urinary samples that arrive in the late afternoon inconvenient and refrigerate them overnight before testing. Furthermore, in certain settings clinics might have difficulty assuring that the urine arrives at the laboratory during the same day as the collection. Because the stability of urine samples for delayed automated dipstick analysis (Supertron) is unknown, after defining precision, we retested urines refrigerated for 24 h to determine stability.

Urinalysis was done twice on the same day and repeated after the sample was refrigerated for 24 h. Combur-10S (Roche Diagnostics) dipsticks were read automatically by a Supertron analyzer. Repeat tests on the same day were compared with tests after storage.

Leukocyte esterase had high precision, but after storage approximately 25% of the positive samples were less reactive (P: <0.005). Precision of hemoglobin retests was also high but declined significantly after storage for 24 h. Urine protein values increased after storage. The precision and stability were excellent for nitrites, glucose, and ketones.

The stability of the automated dipstick urinalysis varies with the substance tested. After refrigeration for 24 h, there is a risk of false-positive results for protein, false-negative results for leukocytes and erythrocytes, and little effect on glucose, nitrite, and ketone values.