Association Between Delayed Sputum Specimen Receipt and Mycobacterium tuberculosis Complex Culture Recovery at the National Tuberculosis Reference Laboratory, Zambia: A Retrospective Laboratory-Based Study, 2025

Abstract Background Mycobacterial culture remains an important method for recovery of viable Mycobacterium tuberculosis complex (MTBC), particularly for confirmation of tuberculosis and drug susceptibility testing. The quality of culture results may be affected by delays between sputum collection and receipt at the culture laboratory. In Zambia, sputum specimens referred for culture are expected to reach the National Tuberculosis Reference Laboratory (NTRL) within seven days of collection. However, the relationship between specimen receipt delay and MTBC culture recovery has not been systematically assessed using routine laboratory data. This study assessed the association between collection-to-laboratory receipt delay and culture outcomes among sputum specimens received at the NTRL. Methods A retrospective laboratory-based observational study was conducted using Laboratory Information System (LIS) data from the NTRL for specimens received from January to December 2025. Patient and specimen characteristics, collection and receipt dates, accessioning dates, and culture outcomes were analysed. Primary culture-outcome analyses were restricted to baseline diagnostic specimens. Specimens received within seven days of collection were classified as on-time, while those received after seven days were classified as delayed. Contaminated cultures were retained as a separate outcome and excluded from the binary MTBC recovery analysis. Logistic regression was used to estimate crude and adjusted odds ratios (ORs) with 95% confidence intervals (CIs) for MTBC recovery. The adjusted model included receipt delay, age group, sex, and HIV status, with robust standard errors clustered at the patient level to account for potential correlation among specimens contributed by the same patient. Receipt-to-accessioning intervals were compared across receipt-delay categories using the Kruskal–Wallis test. Results A total of 1,150 specimens from 712 patients were included. The median patient age was 40 years (IQR: 30–49), and 512 (71.91%) patients were male. Overall, 182 (15.83%) specimens yielded MTBC, 159 (13.83%) yielded non-tuberculous mycobacteria, 697 (60.61%) were negative, and 112 (9.74%) were contaminated. The median collection-to-receipt interval was 4 days (IQR: 2–6), and 999 (86.87%) specimens were received within seven days. Among 573 baseline specimens, 513 (89.53%) were received within seven days and 60 (10.47%) after seven days. After excluding contaminated specimens, MTBC was recovered from 114/457 (24.95%) specimens received within seven days and 8/51 (15.69%) received after seven days. Delayed receipt was associated with lower odds of MTBC recovery in both crude (OR 0.56, 95% CI: 0.26–1.23) and adjusted analyses (aOR 0.51, 95% CI: 0.20–1.26), but neither association was statistically significant. In the adjusted model, MTBC recovery was significantly associated with age group and HIV status, while sex was not statistically significantly associated with recovery. The median receipt-to-accessioning interval was 3 days (IQR: 1–5), with no significant difference across receipt-delay categories (P = 0.695). Conclusion Most sputum specimens reached the NTRL within the recommended seven-day interval. Although MTBC recovery was lower among specimens received after seven days, delayed receipt was not statistically significantly associated with MTBC recovery after adjustment for age, sex, and HIV status and accounting for within-patient correlation. Age and HIV status were associated with MTBC culture recovery, while sex was not statistically significantly associated with recovery. Continued strengthening of timely specimen referral and transportation is warranted, alongside further studies incorporating transport conditions, storage temperature, specimen quality, and other pre-analytical factors.