Sample Rejection Criteria in Clinical Labs: Practical Guide

Sample rejection criteria are the documented conditions under which a clinical laboratory will not test a specimen, or will test it only with a qualifying comment, because the result could be unsafe or misleading. Typical criteria include missing or mismatched identification, the wrong container, insufficient volume, clotting, haemolysis, delayed transport and leaking or contaminated containers.

A clear rejection policy protects patients from wrong results and staff from pressure to test anything that arrives. Applied inconsistently, it frustrates clinicians and invites unsafe exceptions. Below are the usual criteria, how to handle irreplaceable samples and how to track rejections.

Why sample rejection matters

Most laboratory errors originate in the pre-analytical phase, before the sample reaches the analyser. A mislabelled tube can produce a perfectly accurate result for the wrong patient. A haemolysed sample can show a falsely high potassium that triggers unnecessary treatment.

ISO 15189:2022 requires labs to define criteria for accepting or rejecting primary samples, and to record and communicate any acceptance of a sample that does not meet them. It also expects labs to monitor pre-examination performance through quality indicators. For the wider picture, see our pre-analytical errors checklist.

Common sample rejection criteria

Haemolysis, lipaemia and icterus: test-dependent decisions

Haemolysis, lipaemia and icterus do not affect all tests equally. Haemolysis releases intracellular contents, so potassium, LDH and AST can be falsely raised, while some other analytes are barely affected. Lipaemia interferes with some photometric methods, and high bilirubin interferes with others.

Most chemistry analysers measure serum indices (H, L and I) automatically. Use the manufacturer's interference data for each assay, verified where needed, to set index thresholds above which a result is suppressed or reported with a comment. Rejecting every slightly pink sample wastes samples; ignoring indices releases misleading results.

Microbiology-specific criteria

Microbiology results depend on specimen quality as much as method. Common criteria include:

• Sputum that is mainly saliva, shown by many squamous epithelial cells on Gram stain; request a new deep-cough specimen
• Swabs in the wrong medium, such as a dry swab for a test that needs viral or bacterial transport medium
• Excessive delay, such as an unrefrigerated urine for culture that arrives the next day, when overgrowth can mislead counts
• Non-sterile containers for specimens expected to be sterile

Building a rejection policy

A good policy is short, specific and agreed with users. It should include:

• The criteria for each specimen type and test group, with stability limits
• Who can reject and who can authorise exceptions
• How rejection is communicated to the requester, and how quickly
• What is recorded in the LIS and on the report
• Handling of irreplaceable samples
• How rejection data is reviewed

Publish the criteria in the user handbook or collection manual so wards and clinics know them in advance.

Irreplaceable samples: CSF, biopsies and similar

Some samples cannot be recollected without harm or a repeat invasive procedure: cerebrospinal fluid, biopsies, procedural body fluids and some pre-treatment samples. Rejecting them outright may harm the patient more than testing them.

For these, the lab may process the sample while the identification problem is resolved. The person who collected it confirms identity in writing, a senior member of staff authorises testing, and the report carries a documented comment describing the problem. Never release a result until identity is confirmed, and record every exception.

Communicating and recording rejections

Tell the requester promptly, especially for urgent tests. Record the patient, sample, reason, who was informed, when and by whom. Coded rejection reasons in the LIS make trend analysis possible.

Rejections add a full recollection cycle to turnaround time, so the requesting team should know why the sample failed and how to avoid it next time. Where samples move between sites or have legal significance, combine this with chain of custody controls.

Rejection rate as a quality indicator

Track the rejection rate, the number of rejected samples divided by the total received, by reason and by source: ward, clinic, collection centre or phlebotomist. International groups such as the IFCC have published model quality indicators for pre-analytical errors, which can help you define yours consistently.

Look for patterns. A rising haemolysis rate from one clinic usually points to collection technique or transport. Share the data with the source and check that changes work.

Worked example: a haemolysed potassium sample from a Sharjah clinic

A laboratory in Sharjah receives a serum sample from an outpatient clinic for urea, creatinine and electrolytes. The analyser flags a haemolysis index above the threshold for potassium, and the potassium reads high.

Following its policy, the lab suppresses the potassium result and adds a comment that it was not reported due to haemolysis. Creatinine is unaffected at that index level and is reported. The scientist calls the clinic, explains the reason and requests a recollection. Because a genuinely high potassium cannot be ruled out, she suggests the clinician review the patient and repeat the test promptly.

The rejection log shows this clinic has the highest haemolysis rate of the lab's referring sites. A visit finds samples drawn with small-gauge needles, forced from syringes into tubes and carried in a hot car. The clinic switches to evacuated tube collection, staff are retrained and samples travel in an insulated box without direct ice contact. The lab then tracks the clinic's haemolysis rate to confirm the fix.

Common mistakes

• Accepting mislabelled samples because the requester is senior or the test is urgent
• Relabelling tubes in the lab instead of confirming identity with the collector
• Rejecting haemolysed samples for all tests instead of only affected ones
• Rejecting irreplaceable samples without considering a documented exception
• Not telling the requester, so the patient waits for a result that never comes
• Recording rejections on paper with no reasons coded for trend analysis

Key takeaway: A good rejection policy is specific, test-dependent and consistently applied. Every rejection should be communicated, recorded with a reason and used to fix the problem at its source.

Frequently Asked Questions

What are the common specimen rejection criteria?

Common criteria include unlabelled or mislabelled samples, a mismatch between request and sample, the wrong container or additive, insufficient volume, clotted samples for whole-blood tests, significant haemolysis, lipaemia or icterus, delayed transport, wrong temperature, leaking containers and expired tubes. Microbiology adds criteria such as saliva instead of sputum and swabs in the wrong medium.

Should a haemolysed sample always be rejected?

No. Haemolysis affects some tests far more than others. Potassium, LDH and AST are commonly affected, while many other analytes are not significantly affected at low levels. Use serum index thresholds based on the manufacturer's interference data for each assay, suppress affected results with a comment, and report results that remain reliable.

Can a lab relabel a mislabelled sample?

Laboratory staff should not relabel a sample based on assumption. For replaceable samples, request a new collection. For irreplaceable samples such as CSF or biopsies, the collector should confirm identity in writing under a documented procedure, with authorisation and a comment on the report. Follow your lab's policy and local regulations.

How is sample rejection rate calculated?

Divide the number of rejected samples by the total number of samples received in the same period, and express it as a percentage. Break the figure down by reason and by source so you can find patterns. Review it regularly as a pre-analytical quality indicator and include it in management review.

Why is a clotted sample rejected for a full blood count?

Clots trap cells and platelets, so counts can be falsely low and the sample may block the analyser. For coagulation tests, clotting consumes clotting factors and makes results unreliable, so clotted samples should be rejected and a new sample requested.

Next steps

To build sample acceptance into a working quality system, our Laboratory Quality Management System course covers pre-examination processes, quality indicators and improvement. For related reading, see our pre-analytical errors checklist, turnaround time in clinical labs and chain of custody for laboratory samples. If you are not sure where to start, speak to a counsellor.