Examples of Validation Procedures

In the proposed new method, microorganisms suspended in a liquid culture are incubated at 37 °C for 20 min in the absence or presence of an antibiotic, using the same drug concentration as is present in the accepted agar plate method. Respiratory activity (breathing) is then measured by a new electrochemical method. If respiratory activity is 90% of the control measurement, made in the absence of antibiotic, the microorganism is susceptible to the antibiotic. The new method requires only 25 min, and can therefore provide results (and effective treatment regimes) much more rapidly than the accepted method. 

Validation was performed using a common laboratory strain of E. coli with 13 antibiotics possessing different mechanisms of action, using both the accepted and the new method. Comparison of results for each antibiotic allowed classification of the results as follows A true positive shows no growth by the agar plate method and 

TABLE 16.1. Validation Results for the New Qualitative Antibiotic Susceptibility Test 

Antibiotic Agar Plate Result Respiration Result Validation Result 

Nalidixic acid No growth Decrease True positive 

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