In case of obstructive disorders, the flow–volume curve has a concave shape, but this feature is not given due attention. Тhe analysis of the velocity indicators of the respiratory function (such as the peak expiratory flow (PEF) and forced expiratory flows (FEFs)) will significantly expand the diagnostic capabilities of the spirometry method. This paper aims to perform a comparative analysis of the diagnostic strength of the methods of the flow-volume curve assessment by the changes in its shape in patients with obstructive airway diseases to determine the most reliable one. The respiratory function of 540 patients was tested (234 are men (57 [36; 67] years) and 306 are women (59 [44; 69] years)), with the ratio of areas under the actual curve and the predicted curve calculated for each one, as well as the angle formed by the curve; the ratio of the actual FEF (henceforth referred to as FEF) to the predicted FEF, cut-off points to differentiate between obstructive diseases and health. On the basis of these results, we concluded whether the patient’s bronchi were blocked. The results were then compared to the Knudson reference equations, with the test’s operational characteristics calculated compared to the standard. The methods of assessing the angle β and the total concavity of the flow-volume curve have high diagnostic sensitivity (87.8% and 95.6% respectively). The assessment of the area under the curve (AEX-FV) has high diagnostic specificity (88.6%). The results obtained show sufficient diagnostic efficiency of the methods of flow-volume curve estimation by the changes in its shape. However, the use of these methods in isolation from the reference equations does not currently seem reasonable for clinical practice. It appears reasonable to use the reference equations and one of the methods of curve shape assessment together.