Reference — Clinical Skills
Equipment QC & Calibration
How to keep blood gas analyzers, oxygen analyzers, spirometers, and ventilators trustworthy — the QC schedules, calibration points, and Westgard rules that catch a drifting instrument before it changes a patient's care.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Diagnostic and therapeutic respiratory equipment only produces trustworthy numbers when it is calibrated and its performance is verified on a schedule. Quality control (QC) runs known reference materials to confirm an analyzer is still accurate (close to the true value) and precise (reproducible); calibration resets the instrument against known standards. This reference covers the QC and calibration expected of RTs at the point of care.
Accuracy vs. precision.Accuracy is the closeness of a measurement to the true value; precision is reproducibility — low scatter on repeated measurement of the same sample. QC programs exist to catch the loss of either.
Blood Gas Analyzer QC
Levey-Jennings chart
Plots each QC control result over time against the established mean, with lines drawn at ±1, ±2, and ±3 standard deviations (SD). It makes two failure patterns visible at a glance:
- Trend— a gradual drift in one direction across several runs, suggesting a slowly deteriorating component (an aging electrode or a degrading reagent).
- Shift— an abrupt jump to a new level that then persists, suggesting a discrete change (a new reagent lot, recalibration, or an electrode replacement).
Two-point (and one-point) calibration
Blood gas electrodes are calibrated against known values that bracket the clinical range. A one-point calibration adjusts the baseline/offset; a two-point calibration sets both slope and offset — for example, two calibration gases for the PCO₂ and PO₂ electrodes, and two buffers for the pH electrode. Modern analyzers automate this at set intervals.
Regulatory QC (CLIA) requires running multiple levels of control material at defined intervals per manufacturer and regulation. Document and act on out-of-range results before reporting patient values.
Westgard Multi-Rules
The Westgard multi-rules are applied to Levey-Jennings QC data to decide whether a run is in control. The 1:2s rule is a warning only — it triggers inspection using the other rules rather than an automatic rejection.
| Rule | Trigger | Flags |
|---|---|---|
| 1:2s | One control value beyond ±2 SD | WARNING only — inspect, do not auto-reject |
| 1:3s | One control value beyond ±3 SD | Reject — random error |
| 2:2s | Two consecutive values beyond ±2 SD on the same side | Reject — systematic error |
| R:4s | Range between two controls in a run exceeds 4 SD | Reject — random error |
| 4:1s | Four consecutive values beyond ±1 SD on the same side | Reject — systematic error |
| 10x | Ten consecutive values on the same side of the mean | Reject — systematic error (bias) |
Board shortcut: 1:3s and R:4s point to random error; 2:2s, 4:1s, and 10x point to systematic error (bias or drift).
Oxygen Analyzer Calibration
- Calibrate at two points before use (each shift or per policy): 21% (room air) as the low point and 100% oxygen as the high point.
- If the analyzer cannot reach or hold the 100% point, the galvanic fuel cell is likely depleted (or, for a polarographic unit, the battery or membrane needs service) — replace it.
- Oxygen analyzers read oxygen partial pressure, so they are affected by ambient pressure and altitude — calibrate in the environment of use.
Spirometry & PFT QC
- Volume calibration.Verify volume accuracy daily with a 3.00-L calibration syringe; per ATS/ERS standards the daily syringe check must fall within ±3.5% (about ±0.105 L on a 3-L injection — the spirometer’s ±3% accuracy plus the syringe’s ±0.5%).
- Biologic control (“bio-QC”). Periodically test a healthy known subject to detect drift the syringe cannot catch, such as sensor nonlinearity or software changes.
- Also perform leak checks, verify flow linearity across the working range, and confirm the gas analyzers used for DLCO. Report volumes at BTPS (body temperature, ambient pressure, saturated), which requires current ambient temperature and barometric pressure.
Ventilator & Monitor Verification
- Ventilators.Run the manufacturer’s operational verification (self-test) before each new patient — it checks the circuit for leaks, measures circuit compliance for tubing-compressible-volume compensation, and verifies exhaled-volume and alarm function.
- Capnographs and other monitors. Calibrate or verify per the manufacturer’s schedule. A capnograph reading that will not zero, or that does not match a co-oximetry or ABG cross-check, should be recalibrated before it is trusted.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- Cairo JM. Mosby's Respiratory Care Equipment. 11th ed. Elsevier; 2022.
- Malley WJ. Clinical Blood Gases: Assessment and Intervention. 2nd ed. Elsevier Saunders; 2005.