Internal quality control in medical laboratories explained

Internal quality control (IQC) helps a medical laboratory check whether an analytical system continues to perform consistently. It uses control material with expected values to identify changes that may affect patient results.

IQC does not prove that every patient result is correct or replace calibration, external quality assessment, maintenance or professional review. It provides regular evidence of measurement stability and signals when investigation is needed.

An effective process therefore goes beyond “run a control and see whether it passes.” Materials, frequency, targets, charts, review rules, action and records must form one laboratory-approved process.

1. What internal QC measures

Every analytical system has some variation. A stable system will not produce exactly the same control value every time, but its results should vary in a predictable way around an established target.

Internal QC helps the laboratory detect two broad kinds of change:

• Random variation, such as an isolated result that is unexpectedly far from the target.
• Systematic change, such as a sudden shift in results or a gradual trend over several control runs.

Possible causes include reagent deterioration, calibration change, instrument instability, environmental conditions, control preparation or operator error. A QC signal does not identify the cause; it signals that the process needs review.

ISO 15189:2022 sets requirements for quality and competence in medical laboratories. Apply it with current national requirements and the laboratory's approved procedures. In India, verify the latest applicable NABL documents rather than relying on a fixed rule quoted online.

2. Control materials and target ranges

A control should behave sufficiently like a patient sample for the examination being monitored. Laboratories commonly use more than one concentration to observe performance at relevant parts of the measurement range. The number and levels depend on the examination system, risk and applicable instructions.

The QC procedure should define:

• the control material, manufacturer and lot;
• the analytes and examination systems covered;
• the control levels to be used;
• preparation, storage, stability and expiry requirements;
• the assigned or laboratory-established target and variation; and
• the criteria used to decide whether performance is acceptable.

Package-insert or peer-group values may provide starting information, but the laboratory should follow its approved process for establishing or verifying working targets and limits. A new control lot should be assessed before routine use, ideally with an overlap period for comparison with the existing lot.

Do not widen limits simply because a control is frequently rejected. Repeated rejection may show that the method, target assignment, performance specification or QC design needs review.

3. When controls are run

Running QC “before patient testing” does not mean that a control must be tested before every patient sample. For many automated systems, controls are run at defined intervals during operation. For batch-based methods, controls may be included with each analytical batch. Other systems require a different approach.

The laboratory should define control frequency and placement using the manufacturer's instructions, method characteristics, stability, workload, previous performance, risk and applicable requirements. The manufacturer's minimum instructions should not be reduced without a justified and permitted laboratory process.

Additional QC may be required after events that can affect performance, such as:

• calibration;
• maintenance or replacement of a critical component;
• a reagent or calibrator lot change;
• prolonged downtime or restart;
• an instrument alarm or environmental excursion; or
• troubleshooting following an unacceptable control result.

The correct frequency is therefore not a universal number. It is a documented decision for each examination system.

4. Reading daily values and trends

A single control result gives only a snapshot. The greater value of IQC comes from viewing results over time.

For quantitative examinations, laboratories often use a Levey–Jennings chart. It shows the target mean and distances from the mean in standard-deviation units, helping distinguish an isolated event from a developing pattern.

Review both the current point and the recent sequence. A value inside an acceptance limit may still deserve attention if successive results move in one direction or remain on the same side of the mean.

Statistical decision rules can standardise review, but more rules are not automatically better. The laboratory should select rules suitable for the method and intended quality requirement. Poorly chosen rules can miss important error or generate false rejection and unnecessary repeats.

The WHO's Laboratory Quality Management System handbook provides a foundation for laboratory quality systems and QC. Statistical design may draw on controlled professional guidance such as the applicable edition of CLSI C24.

5. Shifts, trends and random variation

A shift is an abrupt change, with successive control values settling at a different level from the established mean. Possible causes include a new reagent or calibrator lot, recalibration or changed instrument response.

A trend is a progressive movement in one direction over successive observations. It may suggest gradual deterioration, drift, changing environmental conditions or another developing issue.

Random variation is less predictable. An isolated extreme value may arise from control preparation, pipetting, aspiration or a transient instrument problem.

These patterns are not diagnoses. Investigation must consider the method, instrument history, reagent and control records, maintenance and operator observations.

6. Responding to unacceptable QC

An unacceptable QC result should trigger the laboratory's documented response. If the validity of patient results may be affected, release of the relevant results should be paused until the issue is assessed.

A practical investigation may check:

• whether the correct control, lot and level were used;
• preparation, storage, stability and expiry;
• data entry, reagents, calibrators and consumables;
• instrument flags, temperatures and environmental conditions;
• recent calibration, maintenance or component changes; and
• patterns in other control levels or related analytes.

Repeating a control until it falls within range is not corrective action. A repeat may form part of a defined troubleshooting sequence, but the original result, repeat, reason and action should remain traceable.

Testing should resume only after the cause has been addressed or the system has otherwise been shown to meet the laboratory's acceptance criteria, with the required authorisation recorded.

7. Assessing patient results

When QC indicates a possible analytical problem, consider patient results from the potentially affected period. Review may begin from the last acceptable QC event, but its scope should reflect the type and likely timing of error.

The assessment may consider the analytes involved, likely direction and magnitude of error, patient-result patterns, repeat measurements, comparison with another verified system and clinical significance. An authorised professional should decide whether results require repeat testing, withdrawal, correction or communication.

Not every QC failure requires every patient sample in the interval to be repeated. Conversely, a later acceptable control does not remove the need to assess results already produced. Document the scope, decision and rationale.

8. Records and management review

Useful QC records show more than a final pass or fail status. Depending on the examination system, the record should connect:

• the date, time, instrument, analyte and operator;
• control material, lot, level and expiry;
• result, target, limits and chart position;
• rule or acceptance-criteria flags;
• reviewer and review time;
• investigation and corrective action;
• linked calibration, maintenance, reagent or equipment events;
• assessment of affected patient results; and
• authorisation to resume routine reporting.

Periodic review can reveal recurring failures, lot problems, method imprecision, excessive false rejection, delayed review or ineffective corrective actions. This turns IQC from a daily task into evidence for management and improvement.

Where a LIMS can help

A LIMS can keep results, lots, targets, charts, review status and corrective actions connected in one traceable workflow. Depending on its verified configuration, it may display trends, apply approved rules, notify staff or link QC events with calibration and maintenance.

Software does not select quality goals, determine QC frequency or replace professional judgement. These decisions must reflect the examination system, manufacturer instructions, approved procedures and applicable requirements.

Note : This article is educational and does not replace the laboratory's controlled procedures, manufacturer instructions, accreditation requirements or professional judgement.