Quality control in pulmonary function testing. A help or a hindrance?

The results of 120 sets of ventilatory capacity, lung volume (LV) and carbon monoxide transfer factor (TCOSB) measurements, 40 sets from each of three pulmonary function laboratories in one city, were examined retrospectively. Vital capacity (VC) was estimated by a forced expiratory manoeuvre (FVC), a relaxed expiratory manoeuvre (VCR) and an inspiratory manoeuvre (VCI) in the three different tests. Differences between VCR and FVC ranged from--88 ml to + 1 400 ml with a mean of + 47 ml. VCI differed from the largest estimate of expiratory VC by--1 800 ml to + 300 ml with a mean of--480 ml. Such differences, which were observed in all three laboratories, affect the calculations in each test and thus the final results quoted. Adequate quality control therefore requires comparison of estimates of VC not only within a test but also between tests. Although more time must be spent with some patients, technical errors and poor patient performance can be distinguished from true changes in pulmonary function, thus better enabling the clinician to assess the relationship between the results and the clinical condition of the patient.