Introduction Accurate workplace spirometry measurement is key to giving workers the best clinical assessment of their respiratory health. We were interested in the underestimation of spirometry that occurs if best practice is not adhered to and the significance of this on assessment of health at work.

Methods 667 stone, brick and foundry workers (with varying spirometry experience), carried out lung function testing as part of a larger cross sectional workplace study. Each performed a minimum of 3 forced expirations. Testing continued until each worker had met the ATS/ERS guidance. The final FEV₁ and FVC recorded was the maximum value attained from 3 technically acceptable blows, and that the two highest FEV₁ and FVC values were within 150 mls. Using the final FEV₁ and FVC for each worker, it was then possible to calculate the underestimate of both measures, had only the first blow, or the maximum of the first two blows, been used for interpretation.

Results 613 of the 669 (91.6%) attained the ATS/ERS criteria based on FEV₁. Analysis of the first actual blow, regardless of technical quality, showed an FEV₁ mean underestimate of 250 mls (median = 80 mls, IQR = 210 mls). If only the first technically acceptable blow had been carried out, the FEV₁ would have been underestimated by a mean of 114 mls (60 mls, 150 mls). If only two technically acceptable blows had been carried out, and the maximum of these used, the FEV₁ would have been underestimated by a mean of 36 mls (0 mls, 50 mls). Similarly, the FVC would have been underestimated by a mean of 131 mls (75 mls, 180 mls) if only the first technically acceptable blow had been used for interpretation. If only two technically acceptable blows were carried out, the FVC would have been underestimated by a mean of 43 mls (0 mls, 50 mls).

Conclusion Non adherence to ATS/ERS lung function testing guidance at work can cause the FEV₁ and FVC to be underestimated by clinically significant amounts.