Spirometry is one of the most fundamental tests of pulmonary function. On the basis of this measurement, obstructive pulmonary pathology may be diagnosed and restrictive disease suspected. The spirogram is a plot of volume against time, taken as a subject breathes out after a full inspiration. The flow-volume loop is an alternative way of looking at the same data. On the flow–volume loop, flow is plotted against volume, without reference to time. Spirometry has historically been measured using a mechanical wedge-bellows spirometer, with the analogue trace depicted on a volume-time graph. Flow-volume loops require electronic processing, so that most devices in current usage are digital and capable of displaying results as either a spirogram or a flow-volume loop. There are contraindications to spirometry, due to the changes in intrathoracic pressure and haemodynamics which occur during what is effectively a Valsalva manoeuvre. A key characteristic of spirometry is reproducibility.