Application-related patterns of diluted urine specimens in routine urine drug testing: A retrospective...

Urine specimen validity testing (SVT) supports interpretation of urine drug testing. Creatinine and specific gravity are core dilution markers, whereas pH and nitrite/oxidant testing provide complementary specimen-integrity information. We evaluated associations of testing application context, season, age, and sex with final adjudicated diluted specimen status in a single-center retrospective cross-sectional specimen-record-level study of urine samples submitted between August 1, 2023, and August 1, 2024. SVT included creatinine, specific gravity, pH, and threshold-based nitrite/oxidant assessment. Diluted specimens were operationally defined as creatinine 4.5-<20 mg/dL with specific gravity 1.001-<1.003, together with final adjudication of predefined borderline low-creatinine records. Multivariable logistic regression included age, sex, season, and testing application context; Firth and analytical-reportability-restricted models were used for sensitivity analysis. Among 24,018 records, 118 (0.49%) were diluted and 132 (0.55%) had any non-valid status, including 13 substituted specimen candidates and one adulterated specimen candidate identified by pH >11. Diluted-specimen proportions were highest in forensic applications (97/13,192 [0.7%]) versus administrative (9/3814 [0.2%]) and clinical applications (12/7012 [0.2%]). Compared with forensic applications, adjusted odds were lower for administrative (OR 0.310, 95% CI 0.155-0.620) and clinical applications (OR 0.233, 95% CI 0.128-0.425; both p < 0.001); age, sex, and season were not significant. Testing application context was associated with diluted specimen status, but these specimen-record-level associations do not establish intentional manipulation. SVT interpretation should integrate core and complementary validity parameters, collection documentation, testing context, and relevant preanalytical information.