EXTERNAL QUALITY ASSESSMENT OF THE SERUM BACTERICIDAL TEST - RESULTS OF A METHODOLOGY INTERPRETATION QUESTIONNAIRE/

Two hundred microbiology laboratories in the UK took part in two separ ate experimental external quality assessment distributions related to the serum bactericidal test (SET). In the first, Staphylococcus aureus NCTC 6571 (vancomycin MIC 1 mg/L), was tested against a human serum c ontaining vancomycin 38 mg/L plus gentamicin 0.5 mg/L. In the second, Streptococcus oralis PAJ 112/4183 (penicillin MBC less than or equal t o 0.03 mg/L) and Streptococcus sanguis PAJ 107/4184 (penicillin MBC = 128 mg/L) were tested against human serum containing penicillin 15 mg/ L. Respondents returned their laboratory results and a questionnaire o n clinical interpretation and technical aspects. Most laboratories (19 4/199, 97.5%) recommend the use of the SET in the management of infect ive endocarditis but only 48 (25.2%) often or always change therapy on the basis of the result. A wide range of interpretative criteria, def initions of bactericidal endpoints and methodologies are used. Perform ance in the first distribution was acceptable for 75% of laboratories but in the second only 34% could identify penicillin tolerance; 34 res pondents reported an SET result of less than or equal to 2 for the tol erant strain, 81 laboratories reported one of greater than or equal to 16. Technical factors related to acceptable performance were: sonicat ion of broth before counting the inoculum; knowing the inoculum size i n cfu/mL; use of a 4-8 h broth culture to make the inoculum; incubatio n of recovery plates for >36 h; use of a calibrated pipette to sample for surviving bacteria; use of measured volumes to add the inoculum. U se of uncalibrated pipettes or standard loops to recover survivors was related to poor performance. Microbiology departments in the UK shoul d review the clinical need to perform the SET in the light of their lo cal circumstances and if they elect to continue to offer this test, re vise their methodologies which could be producing misleading results w hen testing alpha-haemolytic streptococci.