Example of impaired FEV₁ with preserved peak flow

Although often correlated, peak flow and FEV₁ may be uncoupled, with peak flow severely underestimating asthma severity. Above are flow-volume and volume-time loops in a patient with preserved peak flow (peak of flow-volume loop) but profound obstruction (FEV₁/FVC <0.5, FEV₁ 44% predicted, plateau FVC not achieved after 10 seconds).