Objectives Methods Results Conclusions Contact 64 Laboratory testing for Clostridium difficile infection: looking for a good algorithm. K. Vandebroek, N. Graindor, G. De Sutter, G. Coppens, E. Oris Ziekenhuis Oost-Limburg, Genk BSIM 21st Annual congress, 2-3 December 2016 Objectives Methods Detection of toxigenic C. difficile with culture and toxin A/B antigen detection on positive culture is a very reliable but time consuming method, with a long turnaround time (TAT) of at least 48 hours. Therefore we evaluated a new diagnostic algorithm with the aim to maintain test validity and to shorten the TAT. Consecutive fluid stool specimens of patients with suspected Clostridium difficile infection (CDI) were evaluated between December 2014 and February 2015. All samples (107) were tested with the C. DIFF QUIK CHEK antigen test (Alere), a rapid enzyme immunoassay to detect glutamate dehydrogenase (GDH), and cultured on a Clostridium agar plate (BioMérieux) with identification of the suspected isolates on Maldi Biotyper (Bruker). Between March 2015 and April 2016, only GDH positive samples (103) were both tested with the Xpert C. difficile PCR test (Cepheid) and cultured with toxin A/B detection (Quik chek, Alere) on the isolate. Table 2: Toxin antigen detection - culture - PCR on GDH positive samples Table 1: GDH antigen testing compared with culture Table 3: Cost calculation of the classic and the new algorithm. Results Conclusions In the first part of the study (see table 1), we compared the GDH antigen test with culture as a reference method on 107 specimens. A sensitivity of 93% and a specificity of 100% was observed. With a prevalence rate for Clostridium difficile of 14%, the GDH screening had a positive predictive value of 100% and a negative predictive value of 99%. In the second part, 103 positive GDH samples were analyzed prospectively (see table 2). Only 83,5% (86) of them were culture positive. This is in contradiction with previous results, probably due to the low number of GDH positive samples (15) in the first part of the study. Of the culture positive samples, 43% (37) were positive with the toxin A/B antigen test after 48 hours all of which were confirmed with the PCR test. On the culture positive, toxin A/B negative samples (49) after 48 hours of incubation, another 43% (21) were determined positive with PCR. Of the culture negative samples (17), six turned out positive with PCR. On a subset (30) of culture positive specimens, 3 additional samples (10%) were found positive when the toxin A/B antigen test was performed after a culturing period of 72 hours. The toxin A/B detection (Quik chek, Alere) directly on stool samples was not considered for the testing algorithm, because a lack of sensitivity in preliminary observations. With our previous algorithm consisting of culturing C. difficile followed by toxin A/B antigen detection, 43% (37) of the positive cultures were claimed positive for toxigenic C. difficile after 48 hours. A longer incubation period of 24 hours increased the sensitivity of the toxin A/B antigen test by 10%. With the new testing algorithm, stool samples were screened with the sensitive and specific GDH antigen test. If positive, a PCR was performed. Although this new algorithm has an additional laboratory cost, a higher rate of toxigenic C. difficile was detected (62% instead of 36%) and results could be delivered within 24 hours. We claim that this new testing algorithm is more efficient in reducing the risk of nosocomial spread and provides a faster intervention towards the patient. Contact Katrien Vandebroek, M. Sc.: katrien.vandebroek@zol.be 64