We present a critical assessment of qualitative diagnostic calibration (QDC), which claims to provide a relative calibration of respiratory inductive plethysmography during natural breathing (Sackner MA, Watson H, Belsito AS, Feinerman D, Suarez M, Gonzalez G, Bizousky F, and Krieger B. J Appl Physiol 66: 410-420, 1989). QDC computes the calibration factor (K) by considering breaths of constant tidal volume (VT) and provides a criterion to select breaths when VT is unknown. We applied QDC on uncalibrated data constructed from simulated sets of thoracic and abdominal volumes, with a predefined K. As expected, QDC yields a correct K when applied to breaths at constant VT. In breathing at quasi-constant VT, the criterion for breath selection is shown to bias the results toward K = 1. For spontaneous breathing, the calculated K deviates from its predefined value and depends heavily on the selection criterion. We conclude that QDC will only provide a correct calibration factor when applied to an entire set of breaths with constant or quasi-constant VT. More generally, physiological conclusions based on QDC should be critically evaluated on a case-by-case basis.