A central venous catheter (CVC) is placed in the right internal jugular vein. On day 12, blood cultures drawn simultaneously from the CVC and a peripheral vein both grow Staphylococcus epidermidis. CLABSI is suspected. The differential time to positivity (DTP) criterion used to implicate the CVC as the source requires:
- A CVC blood culture grows ≥5-fold more colonies than peripheral culture on quantitative culture
- B Both CVC and peripheral cultures must grow identical organisms but CVC must grow within 24 hours of catheter insertion
- C CVC blood culture becomes positive ≥2 hours BEFORE the peripheral blood culture (central line blood grows faster due to higher concentration of bacteria from intraluminal biofilm) ✓
- D CVC culture must grow at a quantitative threshold of ≥1000 CFU/mL to confirm CLABSI vs. contamination
Explanation
Differential time to positivity (DTP) is a non-quantitative method to diagnose CLABSI using standard blood culture bottles. Paired blood cultures are drawn simultaneously from the CVC hub and a peripheral vein. If the CVC blood culture flagged as positive ≥2 hours before the peripheral culture, it strongly suggests the CVC is the infection source — because bacteria growing in biofilm within the catheter lumen are at much higher concentration in blood drawn through the catheter, allowing the automated incubation system to detect growth earlier. DTP ≥2 hours has sensitivity ~85% and specificity ~91% for catheter-related bloodstream infection. This is preferred in institutions without quantitative blood culture capability. Remove or exchange the CVC if CLABSI is confirmed.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.