A novel approach for quality control of total lung capacity in the clinical pulmonary function laboratory: A study in a veteran population

Objective: Quality control in the clinical pulmonary function laboratory has been well developed for spirometry and diffusing capacity but not for the measurement of TLC. The purpose of the present study was to test two approaches to this problem. First, we compared TLC by body plethysmography (TLC _b) with a value predicted from TLC measured by multibreath helium dilution (TLC_m). Concordance between the measured and predicted values would imply the validity of the measurements. Second, we measured the test-retest variability of TLC_b, TLC_m and TLC measured by single breath helium dilution (TLC_s) to assess the consistency of the measurements. Methodology: We performed a prospective study of 815 veterans. Results: The prediction of TLC_b from TLC_m improved (r² increased from 0.44 to 0.64) when FEV₁/FVC and the difference between TLC_m and TLC_s were added to the model. The coefficient of variation for test-retest of TLC_s, TLC _m and TLC_b were 8.9, 7.1 and 5.4%, respectively. Of all tests, 5.9% were inconsistent based on pathophysiology or measurement error and attributed mostly to TLC_m. Conclusions: Prediction of TLC _b from TLC_m was not sufficiently accurate as to be useful for quality control. Comparison of TLC_s, TLC_m and TLC _b may be useful for determining the internal data validity.