Comparison of sensititre dried microtitration trays with a standard agar method for determination of minimum inhibitory concentrations of antimicrobial agents

Abstract

A total of 222 clinical isolates were used to test the accuracy of Sensititre dried microtitration trays for determining minimum inhibitory concentrations (MICs) of antimicrobial agents. In comparison with an agar dilution technique, 89.5% of all the pairs of results were within one doubling dilution. The 2,420 pairs of MIC results with finite values gave a corresponding figure of 86.8%. Exclusion of sulfisoxazole results, which demonstrated a significant interlaboratory variation in accuracy, raised this value to 89.1%. Very good differentiation of /1-lactamaseproducing strains of Staphylococcus aureus (24 of 24 giving an MIC 2 0.25 ,ug/ ml) and Haemophilus influenzae (3 of 3 giving an MIC 2 32 pug/ml) was obtained with the Sensititre system. This method also clearly distinguished erythromycinresistant S. aureus strains (7 of 7 giving an MIC > 32 ,ug/ml) from the susceptible strains (26 of 28 giving an MIC ' 0.5 pg/ml plus 1 strain at 1.0 ,ug/ml and 1 at 2.0 pg/ml). Sensititre offers an accurate and convenient method of detenmining MICs comparable to those obtained with the agar dilution procedure, with the advantage of an extended shelf life when stored at room temperature.The Kirby-Bauer disk diffusion method is widely used for antimicrobial susceptibility tests, but there are limitations to this method, mainly because of the sensitivity of the test to changes in operator technique and also in the subsequent interpretation of zone diameters. In recent years there has been a move towards a more quantitative method, namely, the measurement of an antimicrobial agent's miniimum inhibitory concentration (MIC). To date this type of susceptibility testing has relied on freshly prepared dilutions of antimicrobial agents in nutrient media or on frozen microtitration trays, either made in the hospital's laboratory (9) or, more recently, brought in from an outside supplier (3). The frozen systems have to be stored at -20°C to retain the potencies of the antimicrobial drugs, and at this temperature the trays have a relatively limited storage life. A microtitration system, Sensititre, is now commercially available; it consists of plates containing dried antibiotic which are stable for 12 months at ambient temperature (data are on file with the manufacturer).A previous study has demonstrated the good perfornance of Sensititre when compared with another broth microdilution procedure, and also the high reproducibility of results obtained in several laboratories (6). This report compares Sensititre with the agar dilution procedure for MIC determinations. Particular emphasis was put on selecting organisms known to be resistant to antimicrobial agents. MATERIALS AND METHODSSensititre plates were supplied by Seward Laboratory, UAC House, Blackfriars Road, London

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