Internal Quality Control in a Diagnostic Laboratory: The Exact Daily Process

Many laboratory owners know that quality control should be performed. But the actual process is often unclear.

What is being tested? How does the analyser know whether QC has passed? What should the technician do when it fails? Is rerunning enough? When should the analyser be calibrated or serviced? And when does the incident become a CAPA?

Internal quality control, or IQC, is the routine process used to check whether an analyser and its testing system are performing reliably before the laboratory depends on them for patient results.

It is not merely a register maintained for inspection. It is an operational decision:

Is this analyser performing well enough for us to trust the results it is producing now?

This article explains that complete process in practical terms for laboratory owners.

What is actually tested during internal QC?

A patient sample has an unknown result. It therefore cannot tell the laboratory whether the analyser is measuring correctly.

For QC, the laboratory uses a specially prepared control material containing substances or components at known or assigned concentrations. The analyser processes this material using the same measurement system used for patient samples.

For example:

• a biochemistry control may contain assigned levels of glucose, creatinine, cholesterol and other analytes;
• a haematology control may contain stabilised material representing expected WBC, RBC, haemoglobin, platelet and other values; and
• different control levels may represent normal, low-abnormal or high-abnormal concentrations.

The analyser measures the control material. The result is then compared with the target and allowable limits defined for that control lot, level, method and analyser.

If the observed values are acceptable, the QC run passes. If any required parameter is outside the applicable acceptance criteria, the run fails for that scope and must be investigated.

This is how the analyser is QC tested: it is asked to measure material whose expected behaviour is already known.

The QC setup that must exist before the first run

Daily QC will be meaningful only when the laboratory has configured the correct relationships.

1. Analyser

QC is analyser-specific. If the same test is performed on two analysers, each analyser needs its own QC history because their performance may differ.

2. QC material and lot

Control materials are purchased in lots. The laboratory should record:

• control-material name and manufacturer;
• lot number;
• expiry date;
• date opened or reconstituted, where applicable;
• storage and stability requirements; and
• the analyser or method with which it is used.

Targets can change when the QC lot changes. A new lot must therefore not silently inherit values belonging to the previous lot.

3. QC level

A material may have multiple levels, such as Level 1, Level 2 and Level 3, or normal, low and high. The laboratory decides which levels are required for each analyser and test according to the method, manufacturer instructions, laboratory procedure and applicable requirements.

The levels are different concentration ranges, not quality grades or steps in a sequence. They are commonly used to check whether the analyser performs reliably across more than one clinically relevant part of its measurement range.

For example, a three-level control may represent:

• L1: low or low-abnormal concentrations;
• L2: normal or mid-range concentrations; and
• L3: high or high-abnormal concentrations.

However, this meaning is not universal. A manufacturer may assign the levels differently, provide only two levels, or use names such as Normal and Pathological. The laboratory must use the target sheet for the exact control material and lot rather than assuming that L1 always means low and L3 always means high.

Each level can contain assigned values for several parameters. For example, one haematology L1 vial can have separate expected ranges for WBC, RBC, haemoglobin and platelets. The analyser measures each parameter, and each result is assessed against the range assigned to that level and lot.

4. Parameter targets and limits

For every parameter covered by the control, the laboratory records the applicable:

• target or mean;
• standard deviation, where used;
• unit; and
• explicit acceptance limits or statistical rules.

The result must be assessed against the correct combination of analyser, control material, lot, level and parameter. A result compared with the wrong lot or unit can create a false pass or false failure.

5. QC schedule

The laboratory defines when QC should be performed. Depending on the analyser and method, this may be at the beginning of the day or shift, at defined intervals, with each batch, after a reagent change, after calibration, after maintenance or whenever analyser performance is suspected.

The schedule belongs to the laboratory's procedure. It should not be assumed from another laboratory or applied universally across all analysers.

6. Calibration schedule

Calibration is related to QC but has a different purpose. Each analyser should have a record of when calibration was completed, when it is next due and why it was performed. Calibration following a QC failure should be linked to that failure and to the post-calibration QC used for verification.

For a broader explanation of the difference between QC, calibration and EQAS, see QC, calibration and EQAS: how they work together.

The exact internal QC process

Step 1: The QC run becomes due

The laboratory follows the configured schedule for that analyser. A digital system may show an upcoming alert before the scheduled time and mark the run as Not run if the required QC is not performed.

“Not run” and “failed” are different conditions:

• Not run means the expected QC was not performed.
• Failed means the control was tested and did not meet the acceptance criteria.

Both need attention, but they describe different risks.

Step 2: The technician confirms readiness

Before running the control, the technician verifies:

• the correct analyser and control material;
• lot number and level;
• expiry and open-vial or reconstituted stability;
• correct storage, thawing, mixing and preparation;
• reagent lot and reagent condition;
• analyser temperature, maintenance status and visible alerts; and
• calibration status.

This simple check prevents avoidable failures caused by expired controls, inadequate mixing, incorrect lot selection or missed maintenance.

Step 3: The control is loaded on the analyser

The control is processed according to the manufacturer's instructions and the laboratory SOP. Depending on the analyser, it may be identified using a QC sample ID, barcode, rack position, control file or analyser-specific selection.

The control should be processed through the routine analytical path as far as the method permits. It should not receive special treatment merely to produce an acceptable value.

Step 4: The analyser measures each parameter

The analyser produces results for the parameters contained in that QC material and level.

For example, a haematology QC run may produce acceptable WBC, RBC and haemoglobin values but an unacceptable platelet value. The laboratory should be able to see which parameter failed. The result should not be reduced to a vague statement that “the machine failed.”

If every required parameter and level passes the configured criteria, the QC occurrence is complete. A routine passing run normally does not need a separate approval queue. It is recorded in history and the laboratory continues its work.

Step 5: The result is recorded manually or received through interfacing

QC results can enter the laboratory system in two ways:

1. Manual entry: the technician records the analyser's QC values against the correct analyser, material, lot and level.
2. Analyser interfacing: the analyser transmits the QC results through the existing interface connection.

Interfacing is valuable because it avoids retyping values and provides an accurate timestamp and source. But it works only when the analyser's output and the laboratory configuration provide enough information to identify the result correctly.

Depending on the analyser, the transmitted message may contain:

• analyser or device identity;
• QC sample or control identifier;
• control lot and level;
• parameter codes;
• measured values and units;
• date and time; and
• analyser flags or status.

Not every analyser sends every field. If the message does not contain the control lot or level, the system must obtain it from a verified configuration or ask the user to confirm it. It should not guess.

The interface should map analyser parameter codes to the laboratory's configured QC parameters, store the raw received values once and evaluate them using the authorised limits. Duplicate messages should not create duplicate QC runs.

Whether the result arrives manually or through interfacing, it should enter the same QC history and follow the same pass/fail process.

How the QC result is evaluated

The most immediate check is whether each value is within the configured acceptance limit. Quantitative QC results may also be plotted on a Levey–Jennings chart and assessed using the laboratory's chosen statistical rules.

This matters because a value can still be inside the outer limit while showing a developing problem:

• a shift occurs when results remain on one side of the mean;
• a trend occurs when results steadily rise or fall;
• increasing scatter may indicate reduced precision; and
• a sudden extreme value may indicate a random error or immediate problem.

The analyser or its software may provide its own QC evaluation. A connected laboratory system can also evaluate the received values against its authorised configuration and show the history across time. The original analyser values and flags should remain available for traceability.

What exactly should happen when QC fails?

The purpose of QC becomes visible only when a failure occurs.

The first failed result must remain in history. It should never be overwritten by a later passing result.

1. Identify the failed scope

The technician first confirms:

• which analyser failed;
• which control material, lot and level were used;
• which parameters failed;
• how far the result was from the expected limit;
• whether other levels passed or failed; and
• whether there is a shift, trend or isolated outlier.

This determines whether the problem is likely to be limited to one control, one parameter or the wider analytical system.

2. Verify the obvious causes

Before recalibrating the analyser, check:

• Was the control mixed and prepared correctly?
• Was the correct lot and level selected?
• Is the control expired or beyond stability after opening?
• Was it stored at the required temperature?
• Is the reagent correct, within expiry and properly loaded?
• Are there bubbles, clots, insufficient volume or contamination?
• Is routine analyser maintenance complete?
• Did the analyser show an error or warning?
• Was there a recent reagent change, calibration, shutdown or service activity?

The result pattern should guide the investigation. A single unexpected value may indicate a preparation or transient issue. Several parameters or levels moving together may point towards reagent, calibration, environmental or analyser problems.

3. Rerun—but do not repeat until something passes

If the initial verification indicates a possible handling or transient problem, the technician may rerun the QC. A fresh vial or freshly prepared material may be used when justified.

The rerun must be created as a new run linked to the original failure. It must not replace the failed record.

A passing rerun is evidence, but it is not permission to ignore the first failure. The technician should record what was checked or corrected and confirm that the result is consistent with the recent QC pattern. Repeatedly testing the same material until one value happens to fall within range is not a valid investigation.

4. Calibrate when the evidence points to calibration

Calibration may be appropriate when:

• the analyser or reagent instructions require it;
• a reagent-lot change requires calibration;
• multiple controls show a systematic shift;
• calibration is due or has become invalid;
• recent maintenance affects the measurement system; or
• troubleshooting indicates that the analyser's measurement relationship needs to be re-established.

Calibration should be recorded against the analyser with its date, reason and relevant lot information.

After calibration, the laboratory must run QC again. This is the post-calibration verification run. Patient testing should not be treated as reliable merely because calibration completed successfully; the subsequent QC must show that the system is performing acceptably.

The complete chain should remain visible:

Failed QC → investigation → calibration → post-calibration QC → pass or further action

5. Schedule service when the problem persists or indicates an analyser fault

Service or engineering support may be required when:

• QC continues to fail after the material, reagent and calibration have been verified;
• the analyser produces repeated flags or mechanical errors;
• precision remains poor;
• probes, pumps, sensors, optics, temperature control or other components are suspected;
• scheduled maintenance cannot restore performance; or
• the manufacturer or service engineer recommends intervention.

The failure, service request, engineer's work and replaced components should be documented. After service, required maintenance or calibration is completed and QC is run again to verify acceptable performance before the analyser returns to routine use.

The chain then becomes:

Failed QC → investigation → service → calibration if required → verification QC → return to use

6. Consider whether patient results were affected

If the QC failure suggests that the analyser may have been unstable before the failure was detected, the laboratory should assess results generated since the last acceptable QC.

The assessment may consider the failed parameters, size and type of error, previous QC trend, patient-result pattern, retained samples and whether reports were already released. Results may need repeat testing or correction according to the laboratory's authorised procedure.

A software system may provide alerts and traceability without technically blocking patient workflows. The laboratory's SOP and authorised personnel remain responsible for deciding whether testing or reporting must stop.

Is rerun, calibration or service itself a CAPA?

Not automatically.

• Rerunning a control is a troubleshooting or verification step.
• Replacing a control vial, cleaning a probe, recalibrating or arranging service is an immediate correction or corrective step.
• CAPA is the documented process of understanding why the problem occurred, preventing recurrence and checking whether the action was effective.

Not every isolated QC failure requires a formal CAPA. A full CAPA is more appropriate when the failure is repeated, systemic, high-risk, linked to an incorrect patient result, associated with an audit finding or evidence that the current process is not working.

A practical QC CAPA process

1. Record the original QC failure and affected analyser, levels and parameters.
2. Contain the risk and identify potentially affected work.
3. Investigate the root cause using the QC history, reagent and control lots, calibration, maintenance, service and operator records.
4. Document the immediate correction, such as fresh control, recalibration or service.
5. Define the longer-term corrective or preventive action, with an owner and due date.
6. Run QC and confirm recovery before returning the analyser to routine use.
7. Review later QC runs to verify that the problem has not recurred.
8. Close the CAPA only when there is evidence that the action was effective.

For example, if calibration was repeatedly missed after reagent-lot changes, performing calibration solves the immediate failure. A meaningful CAPA would also change the process so that the next reagent-lot change creates a calibration requirement and a post-calibration QC check. Effectiveness would be confirmed during subsequent lot changes.

What the laboratory owner should be able to see

A laboratory owner does not need to watch every control being loaded. But the owner should have clear visibility of the process.

For each analyser, the laboratory should be able to see:

• current QC status: upcoming, due, not run, passed or failed;
• last successful QC and the next scheduled QC;
• failed levels and parameters;
• reruns linked to the original failure;
• last calibration and the next due date;
• failure-related calibration and post-calibration QC;
• open service issue or active CAPA; and
• complete historical QC and Levey–Jennings trends.

The operational view should show only what requires attention now. Passed runs, closed CAPAs, completed calibrations and the full investigation trail should remain available in history.

The complete process in one view

Stage	What happens	Expected record
Configure	Link analyser, QC material, lot, level, parameters, targets, limits and schedule	Authorised analyser-specific QC setup
Run	Technician prepares and processes the control	Operator, time, analyser, material, lot and level
Capture	Values are entered manually or received through interfacing	Raw parameter values, units, flags and source
Evaluate	Each required value and applicable pattern is assessed	Pass/fail with failed parameters visible
Pass	Scheduled occurrence completes	Immutable QC history; no additional approval step needed
Fail	Original failure is preserved and investigated	Failed run, checks performed and next action
Rerun	A new linked control run is performed	Original fail plus separate rerun result
Calibrate	Calibration is performed when justified	Reason, date and analyser-specific calibration record
Verify	QC is performed after calibration or service	Linked verification result before return to use
Service	Technical intervention is arranged when required	Service request, action, parts and completion
CAPA	Recurrent or significant failure is investigated systemically	Root cause, actions, owner, due date and effectiveness review

The principle laboratory owners should remember

Internal QC is not complete when the control is run. It is complete only when the result has been understood and the right action has been taken.

A passing result provides evidence that the analyser is performing acceptably at that time. A failed result should create a visible, traceable chain of investigation, action and verification.

The most important rule is simple:

Never make a failed QC result disappear. Preserve it, understand it, correct the cause and prove that the analyser has returned to acceptable performance.

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References and further reading

• World Health Organization: Laboratory Quality Management System handbook
• WHO Laboratory Quality Management System Training Toolkit
• CLSI C24: Statistical Quality Control for Quantitative Measurement Procedures

This article explains a general internal-QC workflow. Every laboratory should define its process according to the analyser and method instructions, its approved SOPs and applicable accreditation and regulatory requirements.