Laboratory equipment
- The BacT-alert machine is a large incubator which holds and processes many blood culture bottles
- The machine will automatically flag positive bottle for further analysis
- Negatives are removed at 5-7 days
- The MALDI-TOF can be used to identify organisms on a positive culture
- It uses mass sectrometry to identify peaks associated with particular micro-organisms
- It does not give sensitivity information but can guide tests to give this info
- Sensitivity testing is done by a number of methods including disc diffusion, E-test and vitek
- Disc diffusion: uses a known organism spread over a plate with a selection of antibiotic-impregnated discs placed on it, zone gives a sensitive or resistant result for each
- E-test: uses a scale of increasing concentration of one antibiotic on a place with the organism; determines the minimum inhibitory concentration
- Vitek machine: contains a cartridge with various concentrations of antibiotics and gives MICs for each, can also recognise patterns of resistance and recommend confirmatory tests
Definitions
- Pre-test probability: the likelihood of the patient having a diagnosis prior to the test
- Post-test probability: the likelihood of the patient having a diagnosis once the test result is available
| Test result |
Patient has disease |
Patient does not have disease |
| Positive |
True positive |
False positive |
| Negative |
False negative |
True negative |
- Sensitivity: the probability a test is positive if the disease is present
- The ideal test would have 100% sensitivity (meaning no false negatives occur)
- Specificity: the probability a test is negative if the disease is absent
- An ideal test with 100% specificity would only be positive in the presence of that disease
- e.g. syphilis serology cross-reacts with other conditions such as SLE, TB, HIV
- Colonisation: the presence of bacteria that are not causing disease
- Often normal flora but may be device-associated
- Contamination: the introduction of micro-organisms into a sample that are not causing disease or present in the body site
- May be flora or environmental
- Infection: the presence of a micr-organism that is causing disease