An investigation into the stability and sterility of citric acid solutions used for cough reflex testing.

Citric acid is used in cough reflex testing in clinical and research settings to assess reflexive cough in patients at risk of swallowing disorders. To address a lack of knowledge in this area, this study investigated the stability and sterility of citric acid solutions. Triplicate solutions of citric acid (0.8 M) in isotonic saline were stored at 4 ± 2 °C for up to 28 days and analysed by high-performance liquid chromatography. Microbiological sterility of freshly prepared samples and bulk samples previously used for 2 weeks within the hospital was determined using a pour plate technique. Microbial survival in citric acid was determined by inoculating Staphylococcus aureus, Escherichia coli, or Candida albicans into citric acid solution and monitoring the number of colony-forming units/mL over 40 min. Citric acid solutions remained stable at 4 °C for 28 days (98.4 ± 1.8 % remained). The freshly prepared and clinical samples tested were sterile. However, viability studies revealed that citric acid solution allows for the survival of C. albicans but not for S. aureus or E. coli. The microbial survival study showed that citric acid kills S. aureus and E. coli but has no marked effect on C. albicans after 40 min. Citric acid samples at 0.8 M remained stable over the 4-week testing period, with viable microbial cells absent from samples tested. However, C. albicans has the ability to survive in citric acid solution if inadvertently introduced in practice. For this reason, in clinical and research practice it is suggested to use single-use aliquots prepared aseptically which can be stored for up to 28 days at 4 °C.

The clinical relevance of the citrate effect on International Normalized Ratio determinations depends on the reagent and instrument combination used.

Multiple studies have shown that the two different citrate concentrations in common use as the anticoagulant in blood collection for haemostasis assays can affect the results obtained with the prothrombin time assay. It is clear from the literature that there is considerable variability in the results obtained using different instrument-reagents combinations, but the clinical relevance of these differences is unclear. Most of the studies have used an optical system for end-point detection. This study reports on the citrate sensitivity using mechanical end-point detection. Using two different reagents, one previously shown to be citrate sensitive on optical systems (Neoplastin CI plus) and a citrate-insensitive reagent (Neoplastin CI), we demonstrate that the effect of using different citrate concentrations (0.105 or 0.129 mol/l) has statistically significant but clinically irrelevant effects on the International Normalized Ratio using a mechanical instrument (STA)-reagent combination (mean percentage difference in results, 1.9 and 3.8% respectively). This demonstrates that the citrate effect is both instrument type and reagent dependent. Every reagent and instrument combination needs to be tested to see whether any citrate effect exists. In a secondary study, it was shown that the international reference rabbit thromboplastin (CRM 149(s)) was not citrate-concentration sensitive.