PathologyLabTraining

The Pre-Analytical Phase: Sample Acceptance, Rejection and Common Errors

13 min read · Published: 2025-01-08 · Updated: 2026-06-08

The pre-analytical phase begins the moment a test is requested and ends when the sample is loaded onto an analyser — and it is where the overwhelming majority of laboratory errors arise. Long before a single result is generated, decisions made at the bedside, on the ward trolley and in specimen reception determine whether that result will be safe to act on. This guide explains the current UK standards for sample acceptance and rejection, the most common pre-analytical errors and how each one corrupts a result, written for biomedical scientists and support staff across Bands 2 to 8.

Why the Pre-Analytical Phase Dominates Laboratory Errors

It is one of the most consistent findings in laboratory medicine that the pre-analytical phase — not the analytical or post-analytical phases — accounts for the great majority of total testing errors. Published surveillance and audit data place the pre-analytical contribution very high, with most studies reporting that it accounts for well over half, and frequently the large majority, of all errors across the total testing process. The analytical phase, by contrast, has become extremely reliable thanks to automation, internal quality control (IQC) and external quality assessment (EQA).

The reason is structural. The pre-analytical phase is the least automated, the most distributed across staff groups and the least directly controlled by the laboratory:

This matters because the laboratory carries responsibility for the total testing process. Under ISO 15189:2022 — the current edition of the international standard for medical laboratory quality and competence, against which UK laboratories are accredited by the United Kingdom Accreditation Service (UKAS) — the laboratory must control pre-examination activities even where they occur outside its physical walls. Specimen reception is therefore not clerical work; it is the laboratory's first and most important quality gate.

ISO 15189:2022 Pre-Examination Requirements

ISO 15189:2022 uses the term pre-examination for what most staff call the pre-analytical phase, and devotes a substantial part of its technical requirements to it. The standard expects the laboratory to define, document and control the activities that precede examination, including the following.

A recurring theme of ISO 15189:2022 is patient impact: when a sample is rejected, the laboratory must consider the clinical consequences of the delay, not simply discard and move on. This is the standard's emphasis on the laboratory as a clinically engaged partner rather than a testing factory.

Positive Patient Identification: The Foundation of Sample Integrity

No downstream control can recover a sample that came from the wrong patient. Positive patient identification (PPID) at the point of collection is therefore the single most important pre-analytical safeguard, and it is the focus of national patient-safety attention through bodies such as Serious Hazards of Transfusion (SHOT).

Positive identification means the patient actively tells you who they are. For conscious patients it requires open questions:

1. Ask the patient to state their full first name and surname. 2. Ask them to state their date of birth. 3. Where local policy requires, confirm a third identifier such as the NHS number or first line of address. 4. Check the verbalised details against the wristband and the request, resolving any discrepancy before drawing blood.

Critically, asking a patient to confirm a name or date of birth that you have read out to them is not positive identification — a confused, deaf or unwell patient may agree to anything. For unconscious or paediatric patients, identity is confirmed from the wristband, ideally with a second checker.

Labelling must follow the same discipline. Specimens are labelled at the bedside, immediately after collection, before leaving the patient — never pre-labelled and never labelled away from the patient. Most NHS laboratories require a minimum of three identifiers on the tube (surname, forename, and a unique identifier such as NHS number or date of birth), plus the date and time of collection and the identity of the collector. Pre-printed addressograph or barcoded labels reduce transcription error but do not remove the duty to confirm identity at the chairside.

Sample Acceptance and Rejection Criteria

Every laboratory must publish acceptance and rejection criteria so that requesters know what will be tested and what will be returned. The table below summarises the criteria common across UK pathology services; local policies always take precedence.

| Criterion | Accept | Reject / query | |-----------|--------|----------------| | Patient identification | Three or more matching identifiers on tube and request | Mismatch, missing identifier, illegible, or no name | | Container type | Correct tube/additive for the test | Wrong additive, expired tube, non-sterile pot for microbiology | | Sample volume | Within stated fill range | Under-filled coagulation tube, insufficient quantity (QNS) | | Sample condition | Intact, leak-free, uncontaminated | Leaking, broken, clotted (where unacceptable), grossly haemolysed | | Timing / stability | Received within the analyte's stability window | Aged sample exceeding stability, delayed transport | | Request completeness | Tests, clinical details, requester identifiable | Missing tests, missing clinical detail where required |

A key distinction is between rejection and query. Some failures (a leaking or unlabelled transfusion sample) mandate outright rejection and recollection. Others may be salvageable with a documented authorisation — for example, a partly haemolysed sample from a difficult bleed may be reported with an interpretive comment if the affected analytes are flagged. Any acceptance of a non-conforming sample must be risk-assessed, recorded and traceable, and the report must make the limitation visible to the clinician.

Common Pre-Analytical Errors and Their Effect on Results

Most pre-analytical errors fall into a small number of recurring categories. Understanding the mechanism of each helps biomedical scientists recognise the signature of a compromised sample.

Haemolysis

Haemolysis — the rupture of red cells releasing their intracellular contents and free haemoglobin into the serum or plasma — is consistently the single most common cause of sample rejection. It arises from difficult venepuncture, too-fine needles, excessive vacuum, vigorous mixing, line draws and delayed or rough transport. Modern analysers report a haemolysis index (HI) from serum indices, allowing objective, automated suppression of affected analytes rather than visual judgement alone.

The effect is analyte-specific. Potassium is the classic example: red cells hold far higher potassium than plasma, so even slight haemolysis can produce a spuriously high (and clinically alarming) result. Lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) rise sharply, and free haemoglobin spectrophotometrically interferes with assays such as bilirubin. Because potassium and LDH are exquisitely sensitive even at low haemolysis indices, most laboratories suppress them above defined HI thresholds rather than attempt mathematical correction.

Clotting

A clot in an anticoagulated sample (EDTA for full blood counts, citrate for coagulation) consumes cells and clotting factors and produces falsely low results — most dangerously a spuriously low platelet count, which can prompt unnecessary investigation or transfusion. Clots form when the tube is under-mixed or filled too slowly. Analysers flag many clots, but small clots can pass undetected, so visual inspection and clot-detection checks remain part of good practice.

Mislabelling and Wrong Blood in Tube (WBIT)

Mislabelling covers missing, illegible, mismatched or incomplete labels. Its most dangerous form is wrong blood in tube (WBIT) — blood from one patient placed in a tube labelled with another patient's details. WBIT is undetectable analytically because the sample is internally consistent; only a discrepancy with historical records or a second independent sample reveals it. In transfusion this can cause a fatal ABO-incompatible transfusion, which is why transfusion laboratories apply a zero-tolerance labelling policy with no amendments and no exceptions, and many require a two-sample (group-check) rule — a confirmatory group on a second, independently collected sample before issuing group-specific blood to a patient with no historical record.

Wrong Tube and Wrong Order of Draw

Selecting the wrong additive, or drawing tubes in the wrong sequence, causes additive carryover. Two common failures:

The recommended UK order of draw minimises carryover: blood cultures first, then citrate (light blue), then serum/SST, then heparin, then EDTA, then fluoride-oxalate (glucose) last. The principle is to draw tubes most vulnerable to contamination before those whose additives would interfere.

Sample Stability and Transport

A correctly collected, correctly labelled sample can still produce wrong results if it degrades before analysis. Stability is the analyte-specific window during which a measurand remains within acceptable limits under defined storage conditions, and ISO 15189:2022 requires laboratories to define and respect these windows.

Practical controls include validated transport boxes, temperature monitoring, defined maximum transit times, and clear user instructions on when refrigeration or rapid transport is essential. When a sample arrives outside its stability window, it should be treated as a non-conforming sample under the laboratory's acceptance criteria.

Building a Pre-Analytical Quality Culture

Because pre-analytical errors largely originate outside the laboratory, reducing them requires the laboratory to influence behaviour it does not directly control. Effective programmes share common features:

Treating specimen reception as a skilled, quality-critical function — rather than a low-status receiving bay — is the cultural shift that distinguishes a safe laboratory.

Frequently Asked Questions

Why does the pre-analytical phase cause most laboratory errors?

Because it is the least automated and most distributed part of the total testing process. Patient identification, tube selection, labelling, mixing and transport are largely manual steps performed by many different staff groups, often outside the laboratory's direct control, so a single human slip can invalidate an otherwise perfect analytical result. Published surveillance consistently shows the pre-analytical phase accounts for the large majority of total testing errors.

What is the difference between rejecting and querying a sample?

Rejection means the sample cannot be tested and must be recollected — for example an unlabelled or leaking transfusion sample, which is rejected with zero tolerance. A query means the sample has a fault that may be salvageable with documented authorisation, such as a partly haemolysed biochemistry sample reported with an interpretive comment suppressing the affected analytes. Any acceptance of a non-conforming sample must be risk-assessed, recorded and made visible on the report.

Why is haemolysis such a problem for potassium results?

Red blood cells hold far more potassium inside them than the surrounding plasma, so when cells rupture during or after collection, intracellular potassium leaks out and falsely raises the measured concentration. Even mild haemolysis can push potassium into an apparently critical range, risking an inappropriate clinical response. This is why laboratories use the analyser's haemolysis index to suppress potassium and other sensitive analytes such as LDH above defined thresholds.

What is wrong blood in tube (WBIT) and why is it so dangerous?

WBIT is blood from one patient placed in a tube labelled with another patient's identifiers. It is dangerous because the sample is internally consistent and therefore undetectable by analysis alone — only a mismatch with historical records or a second independent sample reveals it. In transfusion, WBIT can lead to a fatal ABO-incompatible transfusion, which is why transfusion laboratories apply zero-tolerance labelling and often a two-sample group-check rule for patients with no historical group.

What does ISO 15189:2022 require for the pre-analytical phase?

ISO 15189:2022 (which uses the term "pre-examination") requires laboratories to define and control request information, primary sample collection instructions, sample handling, transport and storage, and documented sample acceptance and rejection criteria, with all samples logged on receipt for traceability. It also requires defined pre-examination storage times that respect analyte stability. A distinctive emphasis of the 2022 edition is considering patient impact when samples are rejected or non-conformities occur.

Why does the order of draw matter when filling blood tubes?

Drawing tubes in the wrong order allows additives to carry over from one tube into the next, corrupting results. For example, EDTA carryover into a biochemistry tube produces a falsely high potassium with a falsely low calcium, and clot-activator carryover into a citrate tube falsely shortens clotting times. The recommended order — cultures, then citrate, serum, heparin, EDTA and fluoride-oxalate last — draws the most contamination-sensitive tubes before those whose additives would interfere.

Further training

The pre-analytical phase underpins every other technical discipline covered across the NHS Laboratory Training hub. To build on this article, explore these closely related guides:

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