3.4 Using controls

A control is included in laboratory tests to check the effectiveness of a test and ensure that reagents and equipment are functioning as expected: for example, a test may not work as expected due to changes in environmental conditions or experimental error. Including a full range of controls is particularly important for qualitative or semi-quantitative tests which are common in microbiology laboratories.

Microbiology laboratories need to maintain a stock of control organisms, with known characteristics, to provide QC for different tests. For example, when checking the reagents used for an oxidase test, laboratories should use a known oxidase positive organism (usually Pseudomonas aeruginosa) and an oxidase negative organism (such as E. coli). Control strains should be obtained from a reliable source, for example the American Type Culture Collection or the UK National Collection of Type Cultures, or other commercial sources. The reference control strains should also be stored, maintained and cultured according to the provider’s instruction. The European Committee on Antimicrobial Susceptibility Testing (EUCAST, 2025) also provide detailed guidance on regular and routine quality control testing of control strains. Control strains are also used to ensure that selective or indicator media have been prepared correctly.