Pharmacoeconomic analysis of microbiologic techniques for differentiating staphylococci directly from blood culture bottles

Differentiating staphylococci in blood cultures is a critical issue, particularly when only one of two cultures is positive by Gram staining for staphylococci. New tests for the identification of Staphylococcus aureus allow faster results and definitive treatment compared to the tube coagulase test interpreted at 24 h (TCT24). These newer tests, peptide nucleic acid fluorescence in situ hybridization (PNA-FISH) and real-time PCR (RT-PCR), offer improved sensitivity at higher cost. Data suggest that the tube coagulase test may be interpreted at 4 h (TCT4) with little loss of sensitivity. The impact of variability in turnaround time, sensitivity, specificity, and cost on comparative cost-effectiveness is unknown. Our aim was to establish the cost-effectiveness of TCT24, PNA-FISH, RT-PCR, and TCT4 for direct identification of staphylococci in blood cultures. Decision analysis comparing these strategies was done from the institutional perspective. Besides test variables, other variables included patient risk factors, empirical treatment, and follow-up cultures. Probability and cost estimates came from the literature and institutional data. Base case estimates were derived from institutional rates of 73% contamination when coagulase-negative staphylococci were identified, 67.6% prevalence of risk factors, and 12.4% prevalence of S. aureus when one of two cultures yielded staphylococci. Sensitivity analysis was done across a range of probabilities and costs. In the base case, TCT4 and TCT24 were more cost-effective than RT-PCR and PNA-FISH ($78 versus $120 versus $165 per patient, respectively). The advantage of TCT4 and TCT24 remained robust upon sensitivity analysis. TCT4 should be further evaluated as a rapid, cost-effective means for identification of S. aureus in blood cultures.