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P21.06 A United Kingdom audit of the laboratory diagnosis of Clostridium difficile infection
Abstracts, 7th International Conference of the Hospital Infection Society, 10–13 October 2010, Liverpool, UK / Journal of Hospital Infection 76S1 (2010) S1–S90
P21.04 Comparison of the analytical sensitivity of three commercial real time PCR assays for the detection of MRSA C. Pope, K. Capaldi. St Georges Hospital, United Kingdom Background: Real time PCR is widely used to detect nasal carriage of MRSA in hospitalised patients. However perforamnce of these assays varies. Aims/Objectives: The aims were to compare the sensitivity and specificity of three commercial real time PCR MRSA assays from diluted culture. The assays compared were Molecular Detection Inc (MDI) Detect Ready MRSA, Becton Dickinson GeneOhm MRSA ACP and Roche LightCycler MRSA Advanced Test. Methods: Both the Detect Ready MRSA kit and the GeneOhm MRSA ACP kit were run on the Cepheid SmartCycler platform while the LightCycler MRSA Advanced test was run on the Roche LightCycler 2.0. A colony from an overnight culture plate was diluted to approximately the limit of detection. This equated to adding approximately 102 –103 CFU per reaction. Fifty strains of staphylococci (15 MSSA, 11 CNS and 24 MRSA) with previously confirmed identification and susceptibility profiles were cultured and for each one colony was serially diluted in TE buffer. 10 ml of this dilution was added to real time PCR reaction tubes for each assay. Results: The sensitivity of the LightCycler MRSA Advanced Test, Detect Ready MRSA and GeneOhm MRSA ACP were 66.7%, 96% and 96.3% respectively. Therefore the Roche assay demonstrated the lowest sensitivity. The specificity of the LightCycler MRSA Advanced Test, Detect Ready MRSA and GeneOhm MRSA ACP were 100%, 100% and 96.3% respectively. The BD assay detected one resistant CNS as MRSA. Conclusions: We have shown that the GeneOhm MRSA ACP assay and the Detect Ready MRSA kit have a higher analytically sensitivity than the LightCycler MRSA Advanced Test. P21.05 Two step algorithm for the detection of Clostridium difficile from stool samples N. Clark1 , M. Doughton1 , A. Karas2 , D. Enoch1 . 1 Peterborough & Stamford Hospitals NHS Foundation Trust, United Kingdom; 2 Health Protection Agency, United Kingdom Background: C. difficile is the commonest cause of hospital acquired infectious diarrhoea in the developed world. Rapid and accurate diagnosis is essential. Aim: We sought to determine the optimal testing strategy in terms of accuracy and cost effectiveness. Methods: We prospectively analysed 322 diarrhoeal samples from consecutive patients by enzyme immunoassay (EIA; VIDAS), glutamate dehydrogenase (GDH) and cell cytotoxicity (CTN; HPA). We retrospectively used PCR (GeneXpert) on samples with a positive result by any method. CTN was used as the gold standard. Cost analysis was performed using manufacturer’s quotes (EIA = £5.50; GDH = £4; PCR = £25.35 per test). Testing 20 samples per day took 60 min (EIA), 80 min (GDH) and 40 min (PCR) by a band 4 (£10 per hour). Results: 11 of 322 samples (3.4%) were positive by CTN. EIA detected 12, 304 were negative and 6 equivocal. The sensitivity (sens), specificity (spec), positive predictive value (PPV) and negative predictive value (NPV) were 70%, 98%, 59% & 98%. 44 were positive by GDH (sens 100%, spec 89%, PPV 25% & NPV 100%). PCR was performed on 106 samples. 17 were positive; 3 were invalid. PCR had a sens, spec, PPV & NPV of 100%, 90%, 41% and 100%. Using a 2 step algorithm, the sens, spec, PPV and NPV were 70%, 94%, 78% and 91% for GDH/EIA and 100%, 77%, 47% and 100% for GDH/PCR. EIA was the cheapest option (£1932 for 322 tests) followed by GDH/PCR (£2631) and PCR alone (£8269).
S63
Discussion: Using the 2 step GDH/PCR algorithm would have detected 15 cases, all of which had clinical evidence of infection. 3 of these additional cases were positive by EIA on subsequent samples. The low prevalence of C. difficile in this study affects the PPV and NPV of all tests in this study. In terms of cost analysis, EIA remains the cheapest with PCR being the most expensive. The GDH/PCR algorithm is fast, accurate and cost effective. P21.06 A United Kingdom audit of the laboratory diagnosis of Clostridium difficile infection R. Cooke1 , A. Galloway2 , A. Collins2 , D. Holland2 , G. Trigg2 . 1 University Hospital Aintree, United Kingdom; 2 Freeman Hospital, United Kingdom Background: A consistent UK approach to the laboratory diagnosis of Clostridium difficile infection (CDI) is critically important especially as surveillance is now linked ro national and local targets on CDI reduction. However there have been previous concerns about the ability of laboratories to comply with national standards on CDI diagnosis. A national survey in England found that 19% of microbiology laboratories reported toxin-positive results from non-diarrhoeal samples. Aims/Objectives: To undertake a UK wide audit of the laboratory diagnosis of CDI. Such an audit should help to improve the quality of data fed into the national CDI surveillance programme. Methods: The audit was undertaken on behalf of the Association of Clinical Pathologists (ACP) and the Royal College of Pathologist (RCP), in collaboration with the National Pathology Benchmarking Service of Keele University. A tick-box format questionnaire was posted to all consultant Microbiologists listed on the ACP and RCP databases as working in the UK. Audits standards were based upon published recommendations from the Health Protection Agency and the Department of Health. Results: A total of 200 questionnaires were sent out of which 80 were returned and suitable for analysis. There was considerable variation in laboratory practices. In particular, only 56% of laboratories gave guidance on follow up testing of toxin negative samples, only 68% provided a 7 day week diagnostic service and only 66% would routinely communicate positive in-patient results to ward nursing or medical staff. Conclusions: To ensure a consistent approach to the diagnosis of CDI, UK microbiology laboratories require more detailed and prescriptive guidance than is currently available. P21.07 Evaluation of the application of the Scottish Clostridium difficile infection (CDI) testing protocol in a diagnostic laboratory J. Coia1 , A. Leanord2 , W. Cowan2 , A. Badriya2 , G. McLean2 , D. Nelson3 , C. Wiuff4 , B. Adawi2 . 1 Scottish Salmonella, Shigella & C. difficile Reference Laboratory, United Kingdom; 2 Southern General Hospital, United Kingdom; 3 Glasgow Royal Infirmary, United Kingdom; 4 Health Protection Scotland, United Kingdom Background: The gold standard for Clostridium difficile infection (CDI) diagnosis is cell-culture cytotoxicity. This is not widely available, and has been replaced by immunoassay (EIA) tests. These miss true positives and generate false-positive results. Scottish CDI diagnostic guidance recommends confirmatory testing using a combination of tests in a diagnostic algorithm. Aim: This study had three aims. Firstly, it assessed the use an EIA test (Mini VIDAS) and combined toxin A/B and Glutamate Dehydrogenase (GDH) antigen detection (QuikChek Complete) as part of a diagnostic algorithm. Secondly, it assessed the potential for resolving discrepant results arising from the initial tests by centralised cell-culture cytotoxicity assay at a second laboratory in the same Health Board area. Thirdly, it assessed on-site toxigenic culture to resolve discrepant results.
P21.04 Comparison of the analytical sensitivity of three commercial real time PCR assays for the detection of MRSA C. Pope, K. Capaldi. St Georges Hospital, United Kingdom Background: Real time PCR is widely used to detect nasal carriage of MRSA in hospitalised patients. However perforamnce of these assays varies. Aims/Objectives: The aims were to compare the sensitivity and specificity of three commercial real time PCR MRSA assays from diluted culture. The assays compared were Molecular Detection Inc (MDI) Detect Ready MRSA, Becton Dickinson GeneOhm MRSA ACP and Roche LightCycler MRSA Advanced Test. Methods: Both the Detect Ready MRSA kit and the GeneOhm MRSA ACP kit were run on the Cepheid SmartCycler platform while the LightCycler MRSA Advanced test was run on the Roche LightCycler 2.0. A colony from an overnight culture plate was diluted to approximately the limit of detection. This equated to adding approximately 102 –103 CFU per reaction. Fifty strains of staphylococci (15 MSSA, 11 CNS and 24 MRSA) with previously confirmed identification and susceptibility profiles were cultured and for each one colony was serially diluted in TE buffer. 10 ml of this dilution was added to real time PCR reaction tubes for each assay. Results: The sensitivity of the LightCycler MRSA Advanced Test, Detect Ready MRSA and GeneOhm MRSA ACP were 66.7%, 96% and 96.3% respectively. Therefore the Roche assay demonstrated the lowest sensitivity. The specificity of the LightCycler MRSA Advanced Test, Detect Ready MRSA and GeneOhm MRSA ACP were 100%, 100% and 96.3% respectively. The BD assay detected one resistant CNS as MRSA. Conclusions: We have shown that the GeneOhm MRSA ACP assay and the Detect Ready MRSA kit have a higher analytically sensitivity than the LightCycler MRSA Advanced Test. P21.05 Two step algorithm for the detection of Clostridium difficile from stool samples N. Clark1 , M. Doughton1 , A. Karas2 , D. Enoch1 . 1 Peterborough & Stamford Hospitals NHS Foundation Trust, United Kingdom; 2 Health Protection Agency, United Kingdom Background: C. difficile is the commonest cause of hospital acquired infectious diarrhoea in the developed world. Rapid and accurate diagnosis is essential. Aim: We sought to determine the optimal testing strategy in terms of accuracy and cost effectiveness. Methods: We prospectively analysed 322 diarrhoeal samples from consecutive patients by enzyme immunoassay (EIA; VIDAS), glutamate dehydrogenase (GDH) and cell cytotoxicity (CTN; HPA). We retrospectively used PCR (GeneXpert) on samples with a positive result by any method. CTN was used as the gold standard. Cost analysis was performed using manufacturer’s quotes (EIA = £5.50; GDH = £4; PCR = £25.35 per test). Testing 20 samples per day took 60 min (EIA), 80 min (GDH) and 40 min (PCR) by a band 4 (£10 per hour). Results: 11 of 322 samples (3.4%) were positive by CTN. EIA detected 12, 304 were negative and 6 equivocal. The sensitivity (sens), specificity (spec), positive predictive value (PPV) and negative predictive value (NPV) were 70%, 98%, 59% & 98%. 44 were positive by GDH (sens 100%, spec 89%, PPV 25% & NPV 100%). PCR was performed on 106 samples. 17 were positive; 3 were invalid. PCR had a sens, spec, PPV & NPV of 100%, 90%, 41% and 100%. Using a 2 step algorithm, the sens, spec, PPV and NPV were 70%, 94%, 78% and 91% for GDH/EIA and 100%, 77%, 47% and 100% for GDH/PCR. EIA was the cheapest option (£1932 for 322 tests) followed by GDH/PCR (£2631) and PCR alone (£8269).
S63
Discussion: Using the 2 step GDH/PCR algorithm would have detected 15 cases, all of which had clinical evidence of infection. 3 of these additional cases were positive by EIA on subsequent samples. The low prevalence of C. difficile in this study affects the PPV and NPV of all tests in this study. In terms of cost analysis, EIA remains the cheapest with PCR being the most expensive. The GDH/PCR algorithm is fast, accurate and cost effective. P21.06 A United Kingdom audit of the laboratory diagnosis of Clostridium difficile infection R. Cooke1 , A. Galloway2 , A. Collins2 , D. Holland2 , G. Trigg2 . 1 University Hospital Aintree, United Kingdom; 2 Freeman Hospital, United Kingdom Background: A consistent UK approach to the laboratory diagnosis of Clostridium difficile infection (CDI) is critically important especially as surveillance is now linked ro national and local targets on CDI reduction. However there have been previous concerns about the ability of laboratories to comply with national standards on CDI diagnosis. A national survey in England found that 19% of microbiology laboratories reported toxin-positive results from non-diarrhoeal samples. Aims/Objectives: To undertake a UK wide audit of the laboratory diagnosis of CDI. Such an audit should help to improve the quality of data fed into the national CDI surveillance programme. Methods: The audit was undertaken on behalf of the Association of Clinical Pathologists (ACP) and the Royal College of Pathologist (RCP), in collaboration with the National Pathology Benchmarking Service of Keele University. A tick-box format questionnaire was posted to all consultant Microbiologists listed on the ACP and RCP databases as working in the UK. Audits standards were based upon published recommendations from the Health Protection Agency and the Department of Health. Results: A total of 200 questionnaires were sent out of which 80 were returned and suitable for analysis. There was considerable variation in laboratory practices. In particular, only 56% of laboratories gave guidance on follow up testing of toxin negative samples, only 68% provided a 7 day week diagnostic service and only 66% would routinely communicate positive in-patient results to ward nursing or medical staff. Conclusions: To ensure a consistent approach to the diagnosis of CDI, UK microbiology laboratories require more detailed and prescriptive guidance than is currently available. P21.07 Evaluation of the application of the Scottish Clostridium difficile infection (CDI) testing protocol in a diagnostic laboratory J. Coia1 , A. Leanord2 , W. Cowan2 , A. Badriya2 , G. McLean2 , D. Nelson3 , C. Wiuff4 , B. Adawi2 . 1 Scottish Salmonella, Shigella & C. difficile Reference Laboratory, United Kingdom; 2 Southern General Hospital, United Kingdom; 3 Glasgow Royal Infirmary, United Kingdom; 4 Health Protection Scotland, United Kingdom Background: The gold standard for Clostridium difficile infection (CDI) diagnosis is cell-culture cytotoxicity. This is not widely available, and has been replaced by immunoassay (EIA) tests. These miss true positives and generate false-positive results. Scottish CDI diagnostic guidance recommends confirmatory testing using a combination of tests in a diagnostic algorithm. Aim: This study had three aims. Firstly, it assessed the use an EIA test (Mini VIDAS) and combined toxin A/B and Glutamate Dehydrogenase (GDH) antigen detection (QuikChek Complete) as part of a diagnostic algorithm. Secondly, it assessed the potential for resolving discrepant results arising from the initial tests by centralised cell-culture cytotoxicity assay at a second laboratory in the same Health Board area. Thirdly, it assessed on-site toxigenic culture to resolve discrepant results.