Page created on February 23, 2022. Not updated since.
Complete blood count
In case of an infection, CBC may reveal leukocytosis. In case of infectious mononucleosis, lymphocytosis is typical, with many of the lymphocytes having atypical morphology.
Acute phase reactants
The important acute phase reactants are C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and procalcitonin (PCT) are important in the evaluation of infection. CRP elevation is sensitive for infection but not specific, as it can be elevated in autoimmune disorders, rheumatological disorders, cancer, IBD, etc. Viral infections cause no/small elevation in CRP, while bacterial infections cause a significant elevation. Procalcitonin is more specific, especially for bacterial infections. ESR is the least specific of the three.
These parameters can also be used to monitor therapy. If their level normalises or trend toward normalisation, therapy is likely successful.
Bacterial cultures allow for precise diagnosis of the presence of bacteria, as well as important information to guide antibiotic choice (pathogen, antibiotic sensitivity). They should always be obtained before initiating antibiotics, except in life-threatening situations like septic shock. Unfortunately, bacterial cultures take days to develop.
Blood culture is important in case of severe infections to look for bacteraemia (a sign of invasive infection) and to guide antibiotic choice. Bacteraemia may be primary or secondary to severe infections, like osteomyelitis, pyelonephritis, endocarditis.
Urine cultures are important to diagnose urinary tract infection, especially in young infants. Bacterial culture can also be made from throat swab, CSF, joint aspirate, pleural effusion, and other fluids.
Rapid tests are bedside point-of-care tests which can be used to detect the presence of certain antigens. Examples include rapid strep test for streptococcal pharyngitis, rapid tests for COVID-19, and Monospot test for infectious mononucleosis. These are much faster than bacterial cultures.
Serological tests detect immunoglobulins against specific antigens. This is useful for certain viral disorders like toxoplasmosis, viral hepatitis, and syphilis.
Molecular tests like PCR are very useful because they’re specific, sensitive, and rapid, but they’re expensive. They can be used to diagnose infections like COVID-19, HSV, VZV, and viral meningitis.
Urine analysis or urinalysis (I don’t like this word) is important in evaluating for urinary tract infection. Lumbar puncture is important for meningitis and encephalitis. Liver function tests is important for viral hepatitis.
Manual Gram staining and microscopic examination of a sample may be performed, but is rarely used nowadays.