The goal of each prebronchodilator testing set and post-bronchodilator testing set is to achieve a minimum of three acceptable FEY 1 and three acceptable PVC measurements. Note that acceptable FEV1 and acceptable PVC measurements are not necessarily from the same maneuver. The operator must ensure that sufficient time is allowed between maneuvers for the patient to sufficiently recover and agree to perform another maximal maneuver. PVC repeatability is achieved when the difference between the largest and the next largest PVC is <o.150 L for patients older than 6 years of age (86) and <0.100 L or 10% of largest PVC, whichever is greater, for those aged 6 years or younger (8, 87). For FEY 1 repeatability, the difference between the largest and the next largest FEV1 is <o.150 L for those older than 6 years of age and <0.100 Lor 10% of the largest FEY 1, whichever is greater, for those aged 6 years or younger. If these criteria are not met in three maneuvers, additional trials must be attempted, up to eight maneuvers in adults, although more may be done in children (Figure 3).

Achieving repeatable results is the best indicator that the patient performed the maximal FEY 1 and PVC that she or he was capable of doing. The degree of repeatability, which is quantified in the grading system (see GRADING THE QUALITY OF THE TEST SESSION), guides the confidence level in the interpretation of the results. The repeatability criteria are used to determine when more maneuvers are needed. In many cases, patients can and will achieve closer repeatability than these criteria. Studies have found that most adults are able to achieve PVC and FEY 1 repeatability to within 0.150 L (82, 88) and that children are able to achieve repeatability within 0.150 or 0.100 L (89).

Although an invalid record can show a subject to be in the normal range, it could be a false negative, or if in the abnormal range, a false positive. When using spirometry data in a surveillance program where data is trended to determine if function is declining at an abnormal rate, even though the data may yet still be in the “normal” range, it is vital to get the absolute true values, which is represented by a valid test. Otherwise, the trends may be obscured.