Impact of Shipping Transit Time on Central Laboratory Processing of Total Colony Forming Units (CFU) and Staphylococcus aureus Detection

Abstract

Background: Central laboratory processing of anesthesia work area reservoir samples is used to improve infection control measures. Reservoir samples returning ≥ 100 colony forming units (CFU) and Staphylococcus aureus (S. aureus) detection are monitored to identify improvement targets. The impact of sample shipment time under ambient conditions on these meaningful outcomes has not been characterized. Such insight could help to further optimize feedback that has been proven to generate substantial reductions in surgical site infections. In this study, we aimed to assess the impact of ambient shipping conditions on patient intravenous stopcock sample CFU ≥ 100 and S. aureus detection because stopcock contamination is repeatedly associated with increased patient mortality.

Methods: We conducted a retrospective analysis involving seven geographically dispersed hospitals over a 4.2-year (October 1, 2018 to December 31, 2022) study period. We chose geographically dispersed sites considering variation in ambient shipping conditions and time. Stopcocks sampled at the end of surgery were shipped to a central laboratory, plated to sheep's blood agar, incubated for 24hr at 36°C, CFU/mL quantified, and distinct isolates assessed by colony morphology, Gram stain, simple rapid tests (e.g., coagulase, oxidase, lactose fermentation, catalase), and selective growth medium.

Results: A total of 969 stopcock samples were analyzed. The percentage of stopcocks with CFU ≥ 100 was stable following sample collection from days 3 to 32 (odds ratio (OR) 1.0086/day, 95% confidence interval (CI) 0.9868-1.0309/day) and from sample kit preparation from days 3 to 143 (OR 1.0044/day, 95% CI 0.9991-1.0099/day). S. aureus detection decreased beyond 14 days from the period of collection during the surgical procedure (P = 0.0024; OR 0.83, 95% CI 0.21-0.71).

Conclusions: When utilizing a central laboratory for processing anesthesia workspace reservoir stopcock set samples, there is stability of ≥ 100 CFU for up to 32 days from collection and up to 143 days from kit preparation. S. aureus detection remains stable for up to 14 days. Therefore, when monitoring stopcock contamination to provide feedback, samples should be processed within 14 days from their collection. Anticipated shipment times should be considered by sample collection personnel to ensure optimal sample yield.

Keywords: central clinical laboratory; colony forming units (cfu); detection; s aureus; shipment stability; transit time.

Figure 1. Association between colony forming units (CFU) ≥ 100 and days from surgery to plating to 5% sheep's blood agar (SBA).

From a total of 969 samples (Table 1), 945 were included in this analysis because 24 samples were not exposed to ambient shipping conditions; rather, they were transported directly to the laboratory via use of a currier. The error bars show 99% two-sided Clopper-Pearson confidence intervals, 99% used because of the multiple bars. All bars in the chart include ≥ 102 stopcocks in the denominator and ≥ 32 stopcocks in the numerator, with one exception. The bar shown on the horizontal axis as extending from 33 to 43 days, shown as such only for clarity of presentation, summarizes the leftover counts of 80 stopcocks with observed times from collection on the day of surgery to initial plating to SBA from 33 to 490 days (median time of 224 days). With an alternative threshold for each bar involving a denominator ≥ 75 CFU, the numerators did not meet the criteria of ≥25 CFU (i.e., far wider confidence intervals).

Figure 2. Association between colony forming units (CFU) ≥ 100 and days from kit preparation to plating to 5% sheep's blood agar (SBA).

This figure depicts the association between stopcock CFU ≥ 100 and the number of days from preparation of the collection kit to plating to 5% sheep's blood agar (SBA). The error bars show 99% two-sided Clopper-Pearson confidence intervals, with 99% used because of the multiple bars. All bars in the chart include ≥ 101 stopcocks with CFU ≥ 1 in the denominator and 32 stopcocks in the numerator, with one exception. The bar summarizing 144 to 200 days (horizontal axis) includes leftover counts with observations up to 585 days. When the alternative threshold for each bar was a denominator ≥ 75 CFU, the numerators did not meet the criteria of ≥ 25 CFU (i.e., far wider confidence intervals).

Figure 3. Days from surgery to laboratory plating.

The error bars show 99% two-sided Clopper-Pearson confidence intervals, with 99% being used because of the multiple bars. All bars in the chart include ≥ 250 stopcocks with ≥ 100 CFU (denominator) and ≥ 10 stopcocks with Staphylococcus aureus detected, with one exception. For clarity of presentation, the bar presented as extending from 21 to 43 days, shown on the horizontal axis and matching Figure 1, included 182 stopcocks with times from collection on the day of surgery to plating spanning 21 to 490 days (median of 31 days). Reducing the sample sizes from ≥ 250 to ≥ 225 or to ≥ 200 stopcocks did not change categories and included the threshold of 14 days, matching the earlier study [11]. When the alternative threshold for each bar was a denominator ≥ 150 stopcocks, the numerators included values of five (i.e., far wider confidence intervals).