4.1 Rapid diagnostic tests

Savoldi et al. (2020) reviewed the use of rapid diagnostic tests (RDTs) in conjunction with close collaboration between clinicians and clinical microbiologists. They found that integrating RDTs into the diagnostic workflow requires knowledge of:

• the local epidemiology of phenotypic resistance
• the local epidemiology of molecular resistance mechanisms, if available
• the general or local empiric treatment guidelines.

RDTs improved the time to result and – despite the initial investment in equipment – saved money in the longer term, particularly if combined with AMS. Timely communication between clinicians and the laboratory enabled the appropriate use of tests and real-time notification of test results. In various studies reviewed by Savoldi et al. (2020), this resulted in the earlier transition from empirical to specific treatment, better patient outcomes and a reduced length of stay in hospital – which consistently resulted in an overall cost saving per patient.

Nucleic acid-based rapid tests are commercially available and are easy to carry out. They can be a rapid way to identify pathogens and antibiotic-resistant genes (ARGs). Often they are multiplexed to look for several pathogens and ARGs at the same time, and may return results in as little as one hour. Examples include the Verigene Gram-positive blood culture nucleic acid test, which can be used directly on blood culture bottles, in conjunction with other diagnostic tests, and the FilmArray™ blood culture identification (BCID) panel, which tests for 24 pathogens and three ARGs associated with bloodstream infections.

For RDTs, as for other tests in microbiology, interpretation by the clinical microbiologist is essential, and the test result must be considered in the context of the clinical information and patient history. Note that some RDTs available to support the diagnosis of infectious diseases do not necessarily identify the pathogen, but may indicate an immune response to infection such as the test for CRP (C-reactive protein). The size of the increase in CRP helps to distinguish bacterial infection from viral infection.

Unlike RDTs available for other infections such as malaria, the RDTs currently available specifically for bacteria still require the sample to be processed in the laboratory, and usually, bacteria have to be isolated and grown on an agar plate first – so it may take 18–24 hours before the result is available. RDTs for bacteria are therefore not yet available for near patient or bedside testing.