What a clinical biomarker actually measures

A biomarker is any measurable substance or characteristic that tells a clinician something about a patient's biology. In laboratory terms, that usually means an analyte in blood, urine or another fluid — a hormone, an enzyme, a protein, a metabolite or a fragment of nucleic acid. The result matters not because it exists, but because it shifts the probability of disease in one direction or another.

It helps to think about what a request is really for. Some tests are diagnostic, used to confirm or exclude a condition in someone with symptoms. Some are prognostic, telling you about likely outcome. Some are predictive, indicating whether a particular treatment is likely to work. Others are purely monitoring tests, tracking response or toxicity over time. A troponin measured to rule out myocardial infarction and a troponin measured daily on a ward during treatment are the same analyte doing very different jobs.

The workhorses of NHS laboratory diagnostics

Most requests reaching a hospital laboratory fall into a handful of well-established families. In clinical biochemistry, the daily volume is dominated by:

• High-sensitivity troponin — the backbone of acute chest pain pathways, interpreted with a repeat sample to look at the rate of change rather than a single number alone.
• NT-proBNP — used to support a diagnosis of heart failure, with age-related decision thresholds rather than one universal cut-off.
• HbA1c — reflecting average glycaemia over roughly two to three months, and used both for monitoring diabetes and, at defined thresholds, for diagnosis.
• CRP — a nonspecific marker of inflammation. Useful for tracking response, poor at telling you the cause.
• Thyroid function tests — typically TSH with free T4, and free T3 only when the picture is unusual.
• Renal and liver profiles — creatinine with a calculated eGFR, plus transaminases, bilirubin and albumin.
• Tumour markers and screening analytes — PSA, CA125 and faecal immunochemical testing, all of which need careful handling in primary care because of their modest specificity.

Haematology, microbiology, immunology and genetics contribute their own markers, and point-of-care devices now deliver blood gases, glucose, INR and infectious disease antigens close to the patient. Each has its own analytical quirks.

Sample handling: where results are won and lost

The pre-analytical phase accounts for the majority of laboratory errors, and it is almost entirely outside the laboratory's control. A perfect assay cannot rescue a poor sample.

• Tube choice matters. Serum, lithium heparin, EDTA and citrate tubes are not interchangeable. Using the wrong tube can render an analyte unmeasurable.
• Fill volume matters. Under-filled citrate tubes alter the blood-to-anticoagulant ratio and produce misleading clotting results.
• Haemolysis matters. A difficult venepuncture or a sample forced through a fine needle releases potassium, lactate dehydrogenase and haemoglobin, inflating several results at once.
• Timing matters. Some analytes degrade quickly at room temperature; others are stable for hours. Delayed centrifugation lets glucose fall and potassium rise as cells continue to metabolise.
• Contamination matters. A drop of EDTA from a full blood count tube carried into a biochemistry tube can produce a strikingly high potassium with a low calcium — a pattern worth recognising.

Practical habits help: minimise tourniquet time, avoid sampling from a limb receiving an infusion, label at the bedside, and send samples promptly by the route the local laboratory specifies. For faecal immunochemical tests, follow the kit instructions on collection and return dates precisely — the sample is only valid within the stated window.

Reference ranges, cut-offs and clinical context

A reference interval is usually derived from a healthy population and covers the central 95%. By definition, one in twenty healthy people sits outside it. That is why a mildly abnormal result in a well patient often means very little, and why serial results in the same individual can be more informative than a single value compared against a population range.

Some analytes use decision thresholds rather than reference ranges. HbA1c at 48 mmol/mol or above supports a diagnosis of diabetes. A rise in high-sensitivity troponin above the assay's 99th percentile, combined with a significant change on repeat testing, drives the acute coronary syndrome pathway. PSA has no clean cut-off at all — it is a continuous risk marker, and interpretation depends on age, prostate volume, previous results and examination findings.

Always ask what the test was requested for. A CRP of 60 mg/L means something different in a fit young adult with a sore throat and in an elderly patient on chemotherapy. Pre-test probability does most of the work; the laboratory result refines it.

Quality assurance and the diagnostic pathway

Behind every reported number sits internal quality control, external quality assessment, method validation and accreditation against recognised standards. Laboratories participate in national schemes that circulate blinded samples and compare performance across sites, and they investigate discrepancies formally.

Turnaround times are a clinical governance matter, not just a logistical one. A troponin result that arrives after the decision has been made is of little value. That is why many trusts separate urgent and routine workstreams, and why electronic requesting with clinical details attached improves the usefulness of what comes back.

Interpreting results in practice

A few habits keep interpretation honest. Compare against the patient's own previous results where they exist. Consider analytical and biological variation before acting on a small change. Check whether the sample quality was flagged. Treat a result that does not fit the clinical picture as a prompt to query the sample, not to accept the number uncritically.

And remember the purpose of the test. Biomarkers support decisions; they rarely make them. The most useful conversations happen when the requesting clinician and the laboratory scientist share the same question — and when both are willing to say that the answer, on this occasion, is not yet clear.