6 | VALUE OF DIAGNOSTICS THROUGHOUT THE PATIENT PATHWAY | 2021
immunosorbent assays ( ELISA ), latex agglutination , and immunoblotting . Tests can provide qualitative or quantitative data and can be deployed as rapid , near-patient tests or as part of large , high-throughput automated platforms .
Molecular diagnostics Molecular diagnostics are based on the detection of nucleic acid sequences ( DNA or RNA ) that are specific to the target disease or pathogen . Common examples include fluorescence in-situ hydridisation ( FISH ) and nucleic acid amplification technology ( NAAT ). FISH was developed in the 1980s and uses a fluorescentlabelled DNA probe that binds to target RNA . This creates a specific pattern of fluorescence that facilitates diagnosis . NAAT involves the extraction of DNA / RNA from a sample , which is exposed to a short DNA / RNA sequence ( a primer ) that is specific for the target being investigated . A polymerase chain reaction ( PCR ) is then performed , which amplifies the target DNA , allowing for quantification . NAAT is easily automated and can be performed with multiple primers at once supporting multiplex PCR . With recent advances and wider use of this technology , multiplex PCR can now be performed faster , at less expense , and on minaturised platforms , facilitating the development of point-of-care , multiplex methods .
IVD for infectious diseases Within infectious diseases and medical microbiology , IVDs have played a central role in the diagnosis and management of infection . IVDs
were used to demonstrate that bacteria cause diseases in humans when Hansen observed Mycobacterium leprae in skin samples taken from patients with leprosy in 1873 . 2 This was closely followed by Koch ’ s demonstration of Bacillus anthracis in the tissue of an anthrax victim in 1876 and the demonstration of disease transmissibility . 3 Koch ’ s work , which relied on the use of IVD , launched the field of medical bacteriology .
Compared to other common medical conditions , such as hypertension or diabetes , the management of infectious diseases is relatively unique . The prescription of antimicrobials to treat infection must consider its impact on the organism causing infection , the patient microbiome , and society . The human body comprises approximately 10 13 cells with a similar number of micro-organisms living on and within it 4 ; often termed the microbiome . Most organisms within the microbiome are harmless and many have an important role in our survival . Exposure to antimicrobials will treat the infecting organism but also disrupt the microbiome , potentially selecting out drug-resistant organisms . Antimicrobial prescribing requires consideration of the interactions between the infecting organism ( the bug ), host , and drug factors as well as the wider impact that treatment may have on society through propagation of antimicrobial resistance . Antimicrobials are used ubiquitously across human healthcare . Most prescribers are not expert in infectious diseases and receive little training in appropriate antimicrobial use . 5 Therefore , it is not surprising that in some cases > 50 % of antimicrobial prescriptions are
Common AST methods
Broth dilution Broth dilution exposes a standardised amount of bacteria to antibiotic in serial dilutions ( two-fold dilutions ). The plates are incubated for 16 – 24 hours . The first well with no visible growth of bacteria is determined as the minimum inhibitory concentration ( MIC ), i . e ., the smallest concentration of drug required to inhibit the visible growth of the organism under standardised laboratory conditions .
Antimicrobial gradient The antimicrobial gradient method ( ETEST ® ) utilises a strip test with an increasing concentration of antimicrobial from one end to the other . The strip is placed onto an agar plate inoculated with the target organism and incubated for 16 – 18 hours . The ETEST ® is quick to set up and can determine organism MIC ( note that some variation might be observed compared with broth dilution ).
Disc diffusion Antimicrobial discs of a known concentration are placed onto innoculated agar and incubated for 16 – 18 hours . The zone of inhibition of growth around the disc ( zone diameter ) is then measured and compared to a reference range . Based on the zone diameter , the organism is labelled sensitive , intermediate , or resistant to the antibiotic .