The Impact of Using Non-Birth Sex on the Interpretation of Spirometry Data in Subjects With Air-Flow Obstruction

BACKGROUND: Sex is an important determinant of lung capacity and function. This study examined the impact of using non-birth sex on the interpretation of spirometry data in transgender subjects with air-flow obstruction.

METHODS: This study was a retrospective analysis of anonymous spirometry data. Eighty adult male and 80 adult female subjects were chosen from the database via random sampling. FVC, FEV₁, and FEV₁/FVC were collected and analyzed. Differences in percent of predicted, Z scores, classification of disease severity, and the incidence of a value migrating above or below the lower limit of normal between sex assignments were examined.

RESULTS: For born male subjects, percent of predicted for FVC and FEV₁ were significantly higher when the female sex was used: 100.5% versus 118.5% and 78% versus 91.5%, respectively (P < .001). FEV₁/FVC Z score was −2.53 for male sex and −2.65 for female sex (P = .004). The presence of obstruction was not affected by sex assignment. Use of non-birth sex moved some FVC and FEV₁ data above the lower limit of normal and improved severity classification in others. For born female subjects, percent of predicted for FVC and FEV₁ were significantly lower when the male sex was used: 102% versus 87.5% and 81.5% versus 70.5%, respectively (P < .001). FEV₁/FVC Z score was −2.17 for female sex and −2.12 for male sex (P < .001). Six born female subjects had their FEV₁/FVC normalized when male sex was used. Use of non-birth sex moved some FVC and FEV₁ data below the lower limit of normal and worsened severity classification in others. In total, using the non-birth sex affected spirometry interpretation in 45% of born male subjects and 70% of born female subjects.

CONCLUSIONS: In transgender subjects with air-flow obstruction, using non-birth sex to calculate predicted spirometry values may have a significant impact on test interpretation and place these patients at risk for misdiagnosis and inappropriate treatment.