Laboratory Medicine
Laboratory Medicine
Laboratory Medicine tests safe test ordering, interpretation, and quality across the preanalytical, analytical, and postanalytical phases. Expect questions on malaria diagnostics, CSF interpretation, haemoglobin electrophoresis, and test performance.
Core rules and concepts
- Most laboratory errors are preanalytical: wrong tube, delayed transport, haemolysis, incorrect timing.
- Sensitivity = true positives / all diseased. Specificity = true negatives / all non-diseased. PPV depends on prevalence.
- Malaria: thick film for detection, thin film for species; HRP2-based RDTs can remain positive for weeks after treatment and may be negative in HRP2-deleted parasites.
- CSF: bacterial meningitis typically shows neutrophils, low glucose (CSF/serum ratio <0.4), and high protein; viral meningitis shows lymphocytes and normal glucose.
- Hb electrophoresis: SS = sickle cell anaemia; AS = trait; SC = haemoglobin SC disease.
Worked example
A 25-year-old febrile patient has a positive malaria RDT but a negative thick film. He was treated for malaria two weeks ago. The RDT is likely detecting persistent HRP2 antigen, not active infection. Repeat microscopy and consider alternative diagnoses; do not automatically treat. If local prevalence is 5%, RDT sensitivity 95%, and specificity 90%, PPV is about 33%-most positives are false.
Traps
- Treating a positive RDT as proof of active malaria.
- Interpreting CSF glucose without a paired serum glucose.
- Confusing sensitivity with positive predictive value.
- Accepting a haemolysed sample for potassium measurement.