Equipment Quality Control and Calibration
Verification versus calibration
Daily quality control for a spirometer is volume verification with a 3-L calibration syringe, not true calibration of the sensor. Push the syringe at three flows (low, medium, high) and require each delivered volume to be within 3 percent or 0.05 L of 3.00 L. Testing the syringe at a single flow hides flow-related error, a frequent exam trap. Record the syringe serial number, date, temperature, and results; perform the check at the start of each testing day and after sensor replacement or cleaning.
Other devices
Plethysmographs require pressure and volume calibration, a leak check, and verification that the shutter and mouth-pressure transducers respond correctly. Gas analyzers for DLCO need two-point calibration with reference gases of known carbon monoxide, helium or methane, and oxygen concentrations, plus a linearity check; the CO analyzer should also be zeroed. Verify barometric pressure daily and temperature sensors periodically, because BTPS and STPD corrections depend on them. Inspect pneumotachographs for moisture and debris.
Biological control
A healthy nonsmoking staff member or dedicated bioQC subject performs spirometry periodically, commonly weekly or per laboratory policy, to detect drift that a syringe cannot reveal. Plot the results and investigate shifts greater than the laboratory tolerance.
Worked example
A 3-L syringe check gives 2.85 L, 2.95 L, and 3.05 L at low, medium, and high flow. The low-flow value is 5 percent low, exceeding the 3 percent limit, so the device fails verification. Do not test patients until the sensor is cleaned, recalibrated, and reverified at all three flows.