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

Author(s): Bohn MK, Fabros A, Di Meo A

Publication: Clin Chem Lab Med, 2025, Vol. 63, Page 1169-1178

PubMed ID: 39788127 PubMed Review Paper? No

Purpose of Paper

Conclusion of Paper

Studies

1. Study Purpose

   This study compared semi-quantitative urinalysis (10 analytes test strip) and urine clinical chemistry (13 analytes) results between matched leftover urine specimens that were stored in plain and preservative tubes at room temperature and 2–8°C for up to 96 h. Additionally, the authors investigated the effects of fill volume (25%, 50%, 75% and 100%) and the centrifugation of preservative tubes on semi-quantitative urinalysis results.  This study included leftover random spot urine specimens from 62 patients (no diagnosis specified). To investigate the stability of analytes, urine from 20 patients were aliquoted into plain Sarstedt tubes and urine preservative tubes, and aliquots in each tube type were stored at room temperature and 2–8°C and analyzed after <2 h, 4h, 6 h, 8 h, 24 h, 48 h, 72 h, and 96 h. To investigate effects associated with tube filling, 2, 4, 6 and 8 mL of urine from 10 patients were added to urine preservative tubes to achieve 25%, 50%, 75% and 100% of the total fill volume. To investigate the impact of centrifugation, urine from 10 patients (5 with leukocytes and blood) were analyzed before and after centrifugation at 400 × g for 5 min. Semi-quantitative (ordinal) levels of pH, protein, blood, leukocytes, nitrite, glucose, ketones (acetoacetic acid), bilirubin, urobilinogen, and specific gravity were assayed using reagent cards and a CLINITEK Novus Automated Urine Chemistry Analyzer. Agreement between ordinal results was assessed using Cohen’s kappa coefficient with a minimal acceptable kappa coefficient defined as ≥0.70 and an optimal kappa coefficient defined as ≥ 0.90. Additionally, the percent agreement was calculated.  Quantitative levels of creatinine, urea, protein, albumin, amylase, cortisol, potassium, sodium, chloride, phosphorus, magnesium, calcium, and urate were quantified using an Alinity c analyzer after adjusting the pH to <2 for magnesium, calcium, and phosphorus and to between 8 and 10 for urate. The percent deviation was considered clinically significant when the mean difference or the difference in >50% of specimens was greater than the analyte-specific clinical difference that was calculated based off the analytical variation.

   Summary of Findings:

   The agreement in ordinal (test strip) results for urine specimens stored at room temperature for ≤96 h in plain tubes with baseline results was optimal (κ≥ 0.90) for bilirubin, leukocytes, nitrites, and specific gravity. The maximum storage duration at room temperature for urine specimens in plain tubes for which optimal (κ≥ 0.90) and acceptable agreement were found was lower for blood (24 and 96 h, respectively), glucose (48 h, both), ketones (4 and 96 h, respectively), pH (8 and 24 h, respectively), protein (48 and 72 h, respectively), and urobilinogen (0h and 96 h, respectively). Superior stability in test strip results was observed when urine was stored in plain tubes at 2–8°C compared to room temperature, with optimal agreement observed for all analytes through 96 h, except glucose (optimal ≤4 h, acceptable ≤96 h) and urobilinogen (acceptable ≤96 h). Storage of urine in preservative tubes increased stability, with acceptable or optimal agreement observed for all analytes when stored at either temperature for ≤96 h. However, at the specified storage duration the agreement was only acceptable (not optimal) for blood (≥72 h), ketones (≥8 h), leukocytes (≥72 h), and urobilinogen (≥4 h) when stored in preservative tubes at room temperature , and for glucose when stored in preservative tubes at 2–8°C for ≥8 h. When preservative tubes were underfilled (≤75% of the total fill volume), the agreement in test strip results between properly filled tube and tubes filled 25, 50 or 75% was optimal for all analytes, with the exception of protein and ketones when the tube was filled 25%. However, the percent agreement with properly filled tubes was 70% and 50% for protein in tubes filled 50% and 25%, respectively, and a 70% agreement in ketones in tubes filled 50% or 25%, but the shift was only by a single semi-quantitative level. The agreement for all analytes was optimal between centrifuged and uncentrifuged urine specimens, but the percent agreement was only 70% for leukocytes, with a decrease of one semi-quantitative level following centrifugation.

   Using the quantitative assay, the median percent deviation for all analytes remained within the allowable clinical limit during urine specimen storage at either temperature, but the majority of analytes had some specimens with deviations that exceeded the allowable clinical limit, and variability in levels appeared to increase with storage duration for some analytes. Additionally, there was a trend toward higher creatinine values when storage ≥48 h and toward lower urea and urate when specimens were stored for ≥24 h. While the median percent deviation for all analytes remained within the allowable clinical limit during urine storage in preservative tubes at either temperature, the higher sodium and osmolality in preservative tubes compared to plain tubes exceeded the threshold for clinical significance.

   Biospecimens

   • Bodily Fluid - Urine

   Preservative Types

   • Other Preservative
   • None (Fresh)

   Diagnoses:

   • Not specified

   Platform:

   Analyte	Technology Platform
   Carbohydrate	Clinical chemistry/auto analyzer
   Electrolyte/Metal	Clinical chemistry/auto analyzer
   Small molecule	Clinical chemistry/auto analyzer
   Protein	Clinical chemistry/auto analyzer
   Cell count/volume	Clinical chemistry/auto analyzer

   Pre-analytical Factors:

   Classification	Pre-analytical Factor	Value(s)
   Biospecimen Aliquots and Components	Centrifugation	Centrifuged Not centrifuged
   Storage	Storage duration	<2 h 4 h 6 h 8 h 24 h 48 h 72 h 96 h
   Biospecimen Aliquots and Components	Aliquot size/volume	2 mL (25% fill) 4 mL (50% fill) 6 mL (75% fill) 8 mL (100% fill)
   Storage	Storage temperature	Room temperature 2–8°C
   Storage	Short-term storage solution	Vacutainer urine preservative None
   Biospecimen Preservation	Type of fixation/preservation	Urine preservative tube Refrigeration None (fresh)

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