Assessment of Sample Pooling for Clinical SARS-CoV-2 Testing

Abstract

Accommodating large increases in sample workloads has presented a major challenge to clinical laboratories during the coronavirus disease 2019 (COVID-19) pandemic. Despite the implementation of automated detection systems and previous efficiencies, including barcoding, electronic data transfer, and extensive robotics, capacities have struggled to meet the demand. Sample pooling has been suggested as an additional strategy to address this need. The greatest concern with this approach in clinical settings is the potential for reduced sensitivity, particularly detection failures with weakly positive samples. To investigate this possibility, detection rates in pooled samples were evaluated, with a focus on pools containing weakly positive specimens. Additionally, the frequencies of occurrence of weakly positive samples during the pandemic were reviewed. Weakly positive specimens, with threshold cycle (C_T ) values of 33 or higher, were detected in 95% of 60 five-sample pools but only 87% of 39 nine-sample pools. The proportion of positive samples with very low viral loads rose markedly during the first few months of the pandemic, peaking in June, decreasing thereafter, and remaining level since August. At all times, weakly positive specimens comprised a significant component of the sample population, ranging from 29% to >80% for C_T values above 31. In assessing the benefits of pooling strategies, however, other aspects of the testing process must be considered. Accessioning, result data management, electronic data transfer, reporting, and billing are not streamlined and may be complicated by pooling procedures. Therefore, the impact on the entire laboratory process needs to be carefully assessed prior to implementing such a strategy.

Keywords: COVID-19; enhanced throughput; laboratory capacity; pooling.

FIG 1

Number of specimens tested per day at the Wadsworth Center for SARS-CoV-2, from 29 February to 31 October 2020.

FIG 2

Percentage of specimens in each category of viral load, as approximated by C_T range, by month, from March through October 2020. C_T values are the averages of the N1 and N2 values for each specimen.