Effects of postage on recovery of pathogens from cystic fibrosis sputum samples

Background
Regular surveillance microbiology of sputum is used in cystic fibrosis (CF) to monitor for new pathogens and target treatments. A move to remote clinics has meant greater reliance on samples collected at home and posted back. The impact of delays and sample disruption caused by posting has not been systematically assessed but could have significant implications for CF microbiology.
Methods
Sputum samples collected from adult CF patients were mixed, split, and either processed immediately or posted back to laboratory. Processing involved a further split into aliquots for culture-dependant and-independent microbiology (quantitative PCR [QPCR] and microbiota sequencing). We calculated retrieval by both approaches for five typical CF pathogens: Pseudomonas aeruginosa, Burkholderia cepacia complex, Achromobacter xylosoxidans, Staphylococcus aureus and Stenotrophomonas maltophilia.
Results
93 paired samples were collected from 73 CF patients. Median interval between sample posting and receipt was 5 days (range 1–10). For culture, overall concordance for posted and fresh samples was 86% across the five targeted pathogens (ranging from 57 to 100% for different organisms), with no bias towards either sample type. For QPCR, overall concordance was 62% (range 39–84%), again with no bias towards fresh or posted samples. There were no significant differences in culture or QPCR for samples with short (≤3days) versus extended (≥7days) postal delays. Posting had no significant impact on pathogen abundance nor on microbiota characteristics.
Conclusions
Posted sputum samples reliably reproduced culture-based and molecular microbiology of freshly collected samples, even after prolonged delays at ambient conditions. This supports use of posted samples during remote monitoring.