Problems with real-time PCR for MRSA: why culture is still required

Abstract number: P1966

Dancer S., Coyne M., Morrison D., Speekenbrink A.

Objectives: Real-time PCR processing can detect MRSA from nasal swabs. We describe the introduction of the IDI MRSA test into a clinical laboratory over a six-month period, run in parallel with traditional culture methods.

Methods: One biomedical scientist processed over 2000 nasal swabs using the IDI MRSA test. IDI MRSA positive-culture negative (false positive) lysates were retested using original and modified primers and the results crosschecked against additional MRSA culture findings from patients. IDI MRSA negative-culture positive (false negative) strains were referred for molecular analysis. Total costs of IDI MRSA and routine culture were calculated and compared.

Results: A valid IDI MRSA result was achieved for 75% of swabs tested. About 50% were IDI MRSA and culture negative, 10% were IDI MRSA and culture positive, 0.5% (nine) were IDI MRSA negative-culture positive and 18% were IDI MRSA positive-culture negative. Some 14% samples remained 'unresolved' by IDI MRSA, six of which were culture positive. Repeat testing with modified primers in combination with additional culture results showed that 316 of 362 (87%) 'false positives' were true positives. Seven of nine false negative strains contained mecA, and thirteen of fourteen meticillin-susceptible isolates gave false positive IDI MRSA results, depending upon which preparatory method was used. Sensitivity and specificity of the IDI MRSA method were 93.5% and 60.3%, respectively. Positive predictive value (PPV) was only 37.3%. We recalculated the PPV using additional microbiological data and found that it increased to 90%, with specificity increasing to 74%. Negative predictive value was 99.1%.

Conclusions: The IDI MRSA method was cost neutral compared with routine culture (£14.30 vs £14.80 per test) but results were obtained for only 75% of samples. Further and continued work is needed on the molecular components of this assay. We discuss the reasons for unresolved tests, invalid controls and misleading results. Culture and isolation of MRSA still remains the 'gold standard' where patients are concerned.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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