Comparison of the in-house made Carba-NP and Blue-Carba tests: Considerations for better detection of carbapenemase-producing Enterobacteriaceae

• Carba NP and Blue-Carba are highly sensitive (100%), but Carba NP is more specific than Blue-Carba (98.9% vs. 91.7%).
• Interpretation in both tests is highly subjective, but misinterpretation reduced over time as operators gain experience.
• OXA-48 producers yield clearer results with Blue-Carba test.
• Standardization of the inoculum should be the next step for improving specificity in both tests.

The in-house Carba-NP and Blue-Carba tests were compared using 30 carbapenemase- and 33 non-producing Enterobacteriaceae. Tests were read by three operators. 100% sensitivity was reported for both tests, but Carba-NP was slightly more specific than Blue-Carba (98.9% vs. 91.7%). We describe potential sources of error during tests' preparation and reading.