Understanding what each biomarker measures, the kinetics of its rise and fall, its sensitivity and specificity for bacterial infection versus viral infection versus non-infectious inflammation, and how to use it to guide clinical decisions (antibiotic initiation, antibiotic de-escalation, ICU triage) is practical clinical microbiology and infectious disease knowledge that directly improves patient care.
White Blood Cell Count (WBC) and Differential
The total WBC count is the most basic infection biomarker, but it is also the least specific. WBC is raised (leucocytosis, above 11 x 10^9/L) in bacterial infection, viral infection, physiological stress (including trauma, surgery, and extreme exercise), corticosteroid therapy, malignancy, and many inflammatory conditions. WBC is low (leucopenia, below 4 x 10^9/L) in overwhelming bacterial sepsis, viral infections (particularly influenza, dengue, and viral haemorrhagic fevers), and bone marrow suppression.
The differential white cell count is more informative. Neutrophilia (raised neutrophils, above 7.5 x 10^9/L) is the hallmark of bacterial infection and sterile inflammatory states. Left shift (immature neutrophil forms: bands, metamyelocytes, myelocytes) on the blood film indicates a strong inflammatory stimulus driving emergency granulopoiesis. Lymphopenia (reduced lymphocytes) is characteristic of viral infection and of sepsis (lymphocytes are redistributed and undergo apoptosis in severe sepsis). Eosinopenia (reduced eosinophils, which are normally 0.04 to 0.4 x 10^9/L) is a consistent but under-recognised finding in bacterial infection and sepsis: eosinophil count close to zero in a febrile patient is a subtle indicator of bacterial rather than viral illness.
C-Reactive Protein (CRP): The Versatile Acute Phase Reactant
CRP is a pentameric protein produced by hepatocytes under the stimulation of IL-6, TNF-alpha, and IL-1. It is an acute phase reactant: its serum concentration rises in response to tissue injury, infection, and inflammation. Normal CRP is below 5 mg/L in most laboratories (ultra-sensitive CRP assays detect lower levels for cardiovascular risk assessment but this is not the relevant application for infection biomarker use).
Kinetics: CRP begins to rise 4 to 6 hours after stimulus onset, peaks at 24 to 48 hours, and falls as the stimulus resolves (half-life approximately 19 hours, so falls quickly when inflammation resolves). This makes it a useful marker for monitoring resolution: a falling CRP on appropriate treatment confirms response. A persistently rising or plateau CRP despite treatment may indicate inadequate source control, wrong antibiotic, or development of a complication.
CRP in distinguishing bacterial from viral infection: substantially elevated CRP (above 100 mg/L) is more common in bacterial infection but not diagnostic: CRP is raised in viral infections (particularly influenza, parvovirus B19, adenovirus) and in non-infectious inflammatory conditions (RA flare, inflammatory bowel disease, vasculitis). A CRP below 10 to 20 mg/L is reassuring against severe bacterial infection in an outpatient context, but normal CRP does not exclude bacterial infection (early infection before CRP rise, or localised infection without significant systemic inflammation, can have a normal CRP).
Procalcitonin (PCT): The Bacterial Infection Biomarker
Procalcitonin is the propeptide of calcitonin, normally produced by thyroid C cells and converted to calcitonin. In bacterial infection and sepsis, PCT is produced by virtually all parenchymal cells in response to bacterial lipopolysaccharide (LPS) and other bacterial cell wall components, in a process called "cytokine storm-independent PCT induction."
The key advantage of PCT over CRP: it is more specifically elevated in bacterial infection and falls much faster on successful antibiotic therapy (half-life approximately 24 to 35 hours, consistent with use for antibiotic duration guidance). It is NOT elevated by viral infection alone (viruses do not trigger the LPS/bacterial component-driven PCT induction pathway), though it can be transiently elevated in severe viral infections causing secondary bacterial infection or multi-organ failure.
PCT-guided antibiotic therapy (ProHOSP and PRORATA trials and subsequent meta-analyses): using PCT to guide antibiotic initiation (withholding antibiotics if PCT below 0.25 microg/L in lower respiratory infection) and antibiotic discontinuation (stopping antibiotics when PCT falls by 80 per cent from peak or falls below 0.25 to 0.5 microg/L) reduces antibiotic exposure by approximately 2 days per admission without increasing mortality.
PCT thresholds:
Below 0.1 microg/L: bacterial infection highly unlikely.
0.1 to 0.25 microg/L: bacterial infection unlikely; antibiotic therapy discouraged in lower respiratory infection without clear clinical indication.
0.25 to 0.5 microg/L: possible bacterial infection; antibiotic therapy encouraged based on clinical context.
Above 0.5 microg/L: bacterial infection likely; antibiotic therapy strongly recommended.
Above 2.0 microg/L: high probability of systemic bacterial infection or sepsis.
Limitations: PCT is unreliable in localised infections without systemic involvement (urinary tract infection, soft tissue infection, wound infection) where PCT may remain in the normal range despite genuine bacterial infection. PCT is elevated in cardiopulmonary bypass, major trauma, and some non-infectious systemic inflammatory states. Immunocompromised patients may not mount a normal PCT response.
Lactate: The Sepsis Severity Marker
Lactate is not a marker of infection or inflammation per se: it is a marker of tissue dysoxia and anaerobic metabolism. Elevated serum lactate (above 2 mmol/L) indicates that tissues are not receiving or cannot utilise oxygen effectively, leading to anaerobic glycolysis and lactate production.
In the context of sepsis, elevated lactate is a key criterion for septic shock (Sepsis-3 definition: sepsis plus vasopressors required to maintain MAP above 65 mmHg AND serum lactate above 2 mmol/L despite adequate fluid resuscitation). Lactate above 4 mmol/L is associated with very high mortality in sepsis (greater than 40 per cent in many series). Lactate clearance (fall of 10 per cent or more from initial lactate over 2 hours after resuscitation) is used as a target of early goal-directed resuscitation.
Non-septic causes of elevated lactate: any condition causing inadequate oxygen delivery or consumption (severe anaemia, heart failure, severe respiratory failure) or impaired lactate clearance (hepatic failure) or excessive aerobic lactate production (seizures, severe asthma, intense exercise). Type B lactic acidosis from metformin accumulation in renal impairment is an important non-infective cause in hospitalised patients.
ESR (Erythrocyte Sedimentation Rate): The Long-Duration Marker
ESR measures the rate at which red blood cells settle in a standardised tube over one hour. Elevated acute phase proteins (fibrinogen, CRP, alpha-1-antitrypsin) coat red blood cells and reduce their negative charge, causing increased rouleaux formation and faster sedimentation.
ESR is non-specific and slow: it rises over days to weeks and falls slowly. It is most useful for monitoring chronic inflammatory conditions (temporal arteritis, rheumatoid arthritis, inflammatory bowel disease) and chronic infections (infective endocarditis, bone and joint infection, TB). For acute infection management, ESR is much less useful than CRP or PCT due to its slow kinetics.
Very high ESR (above 100 mm/hour): prompts consideration of serious underlying pathology including multiple myeloma, vasculitis, infective endocarditis, and metastatic cancer, in addition to severe acute infection.
Frequently Asked Questions
What is CRP and what does an elevated level mean?
CRP (C-Reactive Protein) is an acute phase reactant produced by the liver in response to inflammation. An elevated CRP indicates a significant inflammatory response but is non-specific: it is elevated in bacterial and viral infections, non-infectious inflammatory conditions (autoimmune disease, cancer), and tissue injury. Very high CRP (above 100 mg/L) is more suggestive of bacterial infection but cannot confirm it alone.
What is procalcitonin (PCT) and how is it different from CRP?
PCT is the propeptide of calcitonin, produced by all parenchymal cells in response to bacterial cell wall components (LPS). Unlike CRP, PCT is NOT substantially elevated by viral infection alone, making it more specific for bacterial infection. PCT has a short half-life (24 to 35 hours) and falls rapidly on successful antibiotic therapy, making it useful for guiding antibiotic duration.
What PCT level indicates bacterial infection?
PCT above 0.25 microg/L suggests possible bacterial infection and antibiotic therapy should be considered based on clinical context. PCT above 0.5 microg/L suggests bacterial infection is likely and antibiotic therapy is recommended. PCT above 2.0 microg/L indicates high probability of systemic bacterial infection or sepsis. PCT below 0.1 microg/L makes significant bacterial infection highly unlikely.
What is septic shock and how does lactate define it?
Septic shock (Sepsis-3 definition) is sepsis with hypotension requiring vasopressors to maintain a mean arterial pressure of 65 mmHg or higher AND serum lactate above 2 mmol/L despite adequate fluid resuscitation. The lactate criterion distinguishes true septic shock (with inadequate tissue perfusion) from sepsis with hypotension that responds to fluid. Septic shock mortality is approximately 40 per cent.
What does a neutrophil left shift mean?
A neutrophil left shift means the blood contains increased proportions of immature neutrophil forms: band forms, metamyelocytes, and myelocytes. This indicates that the bone marrow is releasing neutrophils prematurely due to high demand from a severe inflammatory stimulus, most commonly bacterial infection or severe systemic inflammation. Left shift is a stronger indicator of bacterial infection than neutrophilia alone.
Can procalcitonin be used to stop antibiotics earlier?
Yes, PCT-guided antibiotic cessation has been validated in multiple randomised controlled trials. When PCT falls by 80 per cent from its peak value, or falls below 0.25 to 0.5 microg/L (depending on the protocol), antibiotics can be safely stopped in most infections without increased mortality. This approach reduces antibiotic exposure by approximately 2 days on average in trials of lower respiratory infection and sepsis.
Is CRP useful for monitoring antibiotic treatment response?
Yes, CRP kinetics are useful for monitoring treatment. A CRP that is falling by 24 to 48 hours of appropriate antibiotic therapy indicates response. A CRP that remains static or continues rising despite antibiotics suggests: wrong antibiotic for the causative organism, inadequate source control (undrained abscess, retained foreign body), or a non-bacterial cause of inflammation (vasculitis, malignancy, drug reaction).
What is the ESR and when is it clinically useful?
ESR (Erythrocyte Sedimentation Rate) measures how quickly red blood cells settle in a standardised tube over one hour. It is a non-specific, slow-rising, slow-falling acute phase marker useful for monitoring chronic inflammatory conditions (temporal arteritis, rheumatoid arthritis, TB, infective endocarditis, osteomyelitis) where disease activity changes over weeks to months rather than days. It is not useful for acute infection management due to its slow kinetics.
What causes a falsely low CRP in bacterial infection?
CRP may be falsely low in: very early infection (before the hepatic acute phase response develops, approximately 4 to 6 hours after stimulus onset), neonatal infection (immature hepatic response), highly immunocompromised patients with poor cytokine response capacity, and in infections that remain localised without significant systemic inflammation (early skin and soft tissue infection without cellulitis, for example).
What is the NEWS score and how does it relate to biomarkers?
NEWS (National Early Warning Score) 2 is a standardised clinical early warning score used in UK hospitals to identify deteriorating patients based on six physiological parameters (respiratory rate, oxygen saturation, temperature, systolic BP, heart rate, and level of consciousness). NEWS is complementary to biomarkers: NEWS captures current physiological deterioration, while biomarkers (CRP, PCT, lactate) reflect the underlying biological driver. A high NEWS score with elevated lactate in the context of suspected infection triggers the Sepsis-6 bundle response.