Calibration of Laboratory Instruments
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Direct answer
Calibration assigns truth; quality control polices it: calibration is the periodic operation that maps the instrument's raw signal to known values using calibrator materials traceable to reference methods or materials (an analyser's factor and slope set against assigned concentrations), while daily controls of known value monitor whether that mapping still holds — and conflating the two is the commonest conceptual error in laboratory medicine examinations. Instruments drift — lamps age, detectors foul, pipettes wear — so the quality system defines when recalibration must occur, and when it must not. Equipment beyond analysers carries its own schedules, because NABL accreditation under ISO 15189 audits the entire measurement chain, not just the assay.
What you must remember
- Definitions with the mark-splitting difference: calibration sets the relationship between signal and concentration using calibrators; quality control verifies it using independent control materials — a control is never a calibrator, and a calibrator never validates routine performance.
- Traceability: calibrator assigned values must be traceable to national or international reference materials or methods (NIST, IFCC, JCTLM-listed), the property that makes results comparable across laboratories and over time.
- When to calibrate: at installation and commissioning; after major service, probe or lamp replacement; at defined intervals (commonly six months for chemistry systems); when controls trend or shift systematically; with reagent lot change where the method demands; and after relocation of the analyser.
- Quality control apparatus: Levey-Jennings charts plotting two-level controls, Westgard multi-rules (1-2s warning; 1-3s, 2-2s, R-4s, 4-1s, 10x rejection rules), and the standing decision tree of troubleshoot-versus-recalibrate.
- Equipment calibration targets: pipettes (gravimetric), balances (certified weights), thermometers and refrigerators (traceable references), centrifuges (speed and timer), spectrophotometers (holmium or didymium wavelength filters; dichromate absorbance), haematology analysers (assigned-value blood), POCT devices per programme.
- Documentation: calibration records, acceptance criteria, the responsible person, corrective actions after failure, anduncertainty-of-measurement awareness — the ISO 15189 audit spine.
- The practical cascade: control fails systematically, troubleshoot first (reagent, control vial, instrument condition); persisting systematic failure demands recalibration — never adjust calibration to fix a random error.
Calibration versus QC
The examination asks the difference, and the complete answer has three parts: what each uses (calibrators with assigned values versus controls with target ranges), what each does (establishes versus monitors the measurement relationship), and independence (controls must not be traceable to the same materials as calibrators — self-validation being the cardinal sin). The second question is the Westgard vocabulary — which rules are warnings (1-2s) and which reject (1-3s random-error flag, 2-2s and 4-1s systematic bias, R-4s range-wide imprecision, 10x sustained shift) — and what the response to each is. The third is the audit question: what an NABL assessor asks for — calibration certificates with traceability, schedules met, failure and corrective-action records, and staff who can state when they would recalibrate and what they would check first. Candidates answering with the decision tree read as bench-trained.
Frequently asked questions
What is the difference between calibration and quality control?
Calibration establishes the signal-to-concentration relationship using traceable assigned-value materials, while quality control monitors that relationship daily using independent control materials of known concentration.
When must an analyser be recalibrated?
At installation, after major maintenance or component replacement, at defined intervals (commonly six months), after reagent lot change where required, and when controls show systematic shift or trend — but not for isolated random errors.
What does traceability of calibrators mean and why does it matter?
Assigned values are linked through an unbroken chain to national or international reference materials and methods, making results comparable across laboratories and over time — the foundation of standardised reporting.
Which Westgard rules reject a run?
1-3s (random error), 2-2s (systematic bias across levels), R-4s (broad imprecision), 4-1s and 10x (sustained shifts); 1-2s serves only as a warning rule prompting review.
Which non-analyser equipment requires calibration in a laboratory?
Balances with certified weights, pipettes gravimetrically, thermometers and refrigerators against traceable references, centrifuge speed and timers, spectrophotometer wavelength and absorbance — all on documented schedules under ISO 15189.