SOFT SALT-MANNITOL AGAR-CLOXACILLIN TEST - A HIGHLY SPECIFIC BEDSIDE SCREENING-TEST FOR DETECTION OF COLONIZATION WITH METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

The early detection of colonization with methicillin-resistant Staphyl ococcus aureus (MRSA of patients in intensive-care units is an essenti al step in the strategy for preventing MRSA epidemics. In this study, tubes containing soft salt-mannitol agar with cloxacillin (6 mu g/ml) (SSMAC) were prepared for inoculation of clinical samples at patients' bedsides by personnel of an intensive care unit. A total of 1,914 swa bs from different sample sites of 81 patients were dipped into SSMAC t ubes, and after 24 h of incubation (in an incubator located near the i ntensive-care unit), an evident color change was considered by the int ensive-care unit personnel to be an MRSA alarm. Sixty-three (3.3%) SSM AC tubes were considered positive for MRSA, 1,827 (95.4%) were conside red negative, and 24 (1.2%) were considered intermediate. Compared wit h values for parallel conventional surveillance cultures for MRSA, exc luding tubes with intermediate results, the SSMAC test had a sensitivi ty of 72.7%, a specificity of 99.2%, a positive predictive value of 76 .2%, and a negative predictive value of 99.0%. When intermediate tubes were considered positive, the corresponding values were 75.3, 98.2, 6 3.2, and 99.0%, respectively. The sensitivity and specificity values o f the test to identify MRSA-colonized patients were 89.4 and 100%, res pectively. Oropharyngeal and naris specimens were the most reliable sa mples for MRSA detection. False-negative results were frequent in bron chial aspirates with low (<10(3) to 10(6) CFU/ml) MRSA counts. False-p ositive results were mainly due to methicillin-resistant Staphylococcu s haemolyticus. The SSMAC tube is a useful, rapid, and inexpensive too l for the early identification of MRSA-colonized patients and, consequ ently, for the implementation of measures to prevent the spread of MRS A.