It is generally assumed that spirometry errors reduce results and that accuracy is guaranteed by frequent spirometer calibration. However, changes occurring in the spirometer’s sensor or calibration during testing or errors in technique can cause elevated results. Our goal is to highlight these often-unrecognized problems using spirograms and test results that illustrate potential errors.
Anomalous spirograms and test results were identified among tests submitted for technical quality review during NIOSH-approved spirometry courses or on a consulting basis from industrial medical departments, occupational medicine clinics, private practices, and hospital pulmonary function laboratories.
Equipment Errors: Using various flow-type spirometers, spurious results were apparently caused by inaccurate zeroing of the spirometer sensor, condensation, mucus deposition, or unstable calibration. Figure 1shows a zero flow error that elevated the FVC to 204% Pred. Figure 2shows possible mucus or condensation on a screen pneumotach which increased the FVC to 144% Pred. Zero errors and changes in pneumotach sensors probably caused 40% of 121 workers in respirator medical clearance programs to record both FVC and FEV1 >120% of predicted.Technique Errors: An extra breath at the end of the maneuver elevated the FVC from 96% to 119% Pred (Figure 3).CONCLUSIONS: Zeroing errors and changes in pneumotach sensors that occur during testing can significantly elevate results recorded on calibrated spirometers. Some errors in technique have a similar effect. Since spirometers report the largest values from a test, these errors must be recognized and deleted as they occur.
Clinicians should recognize these potential errors that substantially elevate test results. Otherwise, these errors may lead to false negative interpretations rather than false positives. QC programs should scrutinize spirograms for technique and equipment errors that elevate spirometry results, even when spirometers are calibrated frequently. Patterns of elevated FVC and/or FEV1 probably indicate spirometer malfunction or errors in testing technique.
M.C. Townsend, None.