Interpreting results
Targeted laboratory investigations will help narrow your differentials for a patient with suspected infection. If investigations are performed promptly at the initial presentation, results will likely be available 24-72 hours later, allowing you to review and amend the management as appropriate (Start Smart then Focus).
Top tip: treat the patient, not investigation results, especially if they don’t fit the clinical picture. When interpreting results, ask yourself, is this consistent with the diagnosis? Investigations can support a diagnosis but should never make the diagnosis.
Haematology
Full blood count
- White blood cell (WBC) count
- Neutrophils usually increase in bacterial infections – but beware of low neutrophil count in severe sepsis
- Lymphocytes may increase in viral infections
- Eosinophils may increase in parasitic infection
- Platelets – acute phase reactant, may rise in infection – but beware of low platelet count in severe sepsis
Clotting
- International Normalised Ratio (INR) – may increase in sepsis and invasive group A Beta-haemolytic Streptococcus infection
ESR
- Erythrocyte sedimentation rate (ESR) may rise in inflammatory conditions (however not specific for infection)
Biochemistry
Urea and electrolyte profile
- Raised urea is a prognostic marker in severe pneumonia (CURB-65 score)
CRP
- Rises in inflammation – peak 24-48 hours after onset and fall in response to antibiotics takes a minimum 24-48 hours
Procalcitonin
- Rises in bacterial infection – peak after 12 hours
Top tip: no laboratory marker is uniquely specific to infection, interpret results in the context of the patient.

Example clinical pathology report of suspected bacterial infection. Reference ranges for tests vary, you should refer to the ranges your laboratory provides when interpreting results.
Microbiology
Samples should be taken from the affected body site (e.g., sputum, urine, synovial fluid, cerebrospinal fluid) for the best chance of identifying a causative organism before starting antimicrobials. During the request, state any antimicrobial therapy that patient is on or which you intend to start, and the laboratory can ensure the appropriate antibiotic sensitivities are released.
Microscopy
- Identifies appearances of microorganism, see Starting an antibiotic tab – beware that prior antibiotic use may alter Gram stain appearance
Culture and sensitivity
- Identifies microorganisms that have grown and susceptibilities of antibiotics, see Starting an antibiotic tab
- Biochemical tests help identify cultured organisms by assessing an organism’s ability to use different substrates, or the presence of certain enzymes (e.g., coagulase/catalase/oxidase)

Example microbiology report of a suspected septic arthritis.
Susceptibility testing
To test how susceptible bacteria is to an antibiotic, we determine the minimum amount of antibiotic that stops the bacteria from growing, the Minimum Inhibitory Concentration (MIC). This can be done by culturing the bacteria in the presence of an antibiotic and determining whether the MIC is above a predetermined ‘breakpoint’ level. Above a certain MIC, bacteria is labelled as resistant (R) to an antibiotic, below a certain MIC value bacteria is labelled as sensitive (S) to an antibiotic.
Susceptibility testing allows us to switch to narrow spectrum agents, and also helps provide epidemiological data to inform local antibiotic and infection prevention and control policies, as well as public health surveillance.
Top tip: most microbiology laboratories will now identify organisms using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF). This uses mass spectroscopy to detect proteins of different masses in a culture specimen to create a signature which is then compared to reference libraries of bacteria and fungi. This improves time from sampling to identification in many cases.
Serology
If culture methods are not appropriate, for example where the organism cannot be easily grown in culture (e.g., Rickettsia, mycoplasma, viruses) serology can be used to look for evidence of infection by detecting antibodies.
Antibody detection
- Rapid result detecting small parts of infecting microorganism, or molecules from infected cells
- Rapid result detecting patient’s response to infection. Interpretation can be complex, but broadly IgM suggests acute/recent infection (however may be present in other conditions that stimulate the immune system), whereas IgG suggests immunity/past infection
- Beware cross reactivity, for example infection with one virus causing a low-level positive IgM in another (e.g., HIV/EBV/CMV). Other inflammatory conditions (and pregnancy) may also cause false positive results
Antigen detection
- Rapid result detecting small parts of infecting microorganism, or molecules from infected cells
Top tip: Enzyme immunoassays (e.g., ELISA – enzyme-linked immunosorbent assay) use antibodies linked to enzymes to detect bacterial antigens or antibacterial antibodies.
Molecular tests – Nucleic acid detection
- Most commonly done using polymerase chain reaction (PCR) to detect microorganism by multiplying DNA or RNA allowing for detection of minute traces of microorganism
- Quantitative methods for nucleic acid detection for an increasing number of infections are now being used including hepatitis B, hepatitis C, CMV and HIV. These allow measurement of viral load and can be used to monitor disease progression/response to treatment
- As well as viruses/fastidious organisms, PCR using targets from bacterial (16S) and fungal (18S) ribosomes can be used to try to detect infection in samples which are culture negative, for example where sampling has occurred post antibiotic administration
Top tip: whole-genome sequencing (WGS) provides information into the genetic basis of bacteria, resistance mechanisms and pathogen evolution. WGS is now being used to develop novel antibiotics, and support surveillance of antimicrobial resistance. The WHO’s Global Antimicrobial Resistance and Use Surveillance System (GLASS) was set up to manage this surveillance and monitors several ‘priority pathogens’ to provide information on early emergence and spread of resistance.
Rapid diagnostic tests
Rapid diagnostic testing may include any of the above techniques, employed in a novel way where the result is delivered more quickly than traditional methods. This may be within the laboratory or in the clinical/near-patient setting (point-of-care). Examples you may encounter include rapid PCR platforms which can deliver results from sampling in often less than an hour, and point-of-care lateral flow testing devices (e.g., for COVID-19).
Top tip: the Xpert® MTB/RIF assay for the detection of Mycobacterium tuberculosis uses an integrated miniature PCR system to obtain results from unprocessed sputum samples within 90 mins, dramatically reducing time to diagnosis when compared to culture based techniques.
As an antimicrobial stewardship intervention, key questions that rapid diagnostics could help answer at the point-of-contact include:
- Differentiating between bacterial or viral infections and reducing antibiotic prescriptions
- Rapidly identifying causative bacteria to allow focused antibiotic therapy
- Identifying bacterial resistance mechanisms to give predicted sensitivity profiles
Knowing these answers allows a clinician to reach the optimal treatment quickly, reducing the risk of misuse of antimicrobials, and as a result rapid diagnostic tests at the point-of-care are becoming more widely used.
Top tip: it is important that quality assurance remains in place to ensure the result is not only quick, but accurate. This can be challenging when testing is taken outside of the controlled laboratory setting. There may be a degree to which accuracy is knowingly compromised for a faster result, and this must be factored into any decision making. For example, when non-molecular rapid tests (e.g., lateral flow for SARS-CoV-2) are used, be aware of the sensitivity and specificity of the test you are using when interpreting the result!
Author: Dr David McMaster (Academic Foundation Doctor) & Dr Owen Seddon (Consultant in Microbiology and Infectious Diseases, Public Health Wales)