In order to diagnose an obstructive ventilation disorder, an individual predicted average value for the FEV1/FVC ratio of the examined person is derived. If the ratio of the determined parameters FEV1and FVC is below the LLN, the diagnosis will be classified as clinically suspicious or pathological and will give rise to further diagnostic procedures. 
If the scatter range of normal values is to be considered, percentiles will be found which establish a relationship between the examination result and its statistic normal distribution in percentage steps. LLN and percentiles can be correlated, so the 5% percentile has been stipulated as the pathological limit of LLN (corresponding to a z-score of -1.645). In the guideline for spirometry (Criée et al, 2015) the severity classification is not recommended in percent of predicted value anymore as it used to be, but a classification according to the z-score. As a criterion for decision in serial examinations GLI recommends GLI the 2.5{^}th^ percentile as LLN. As a criterion for clinical assessment of ill persons the 5{^}th^ percentile is considered acceptable as LLN. The use of LLN as criterion for decision differs from the so-far common practice where e. g. an obstructive ventilation disorder was detected when the FEV1/FVC ratio was inferior to 0.7. A fix limit of 0.7 does not take into account the considerable physiological dependence of the FEV1/FVC ratio on the age of the examined person. Significant differences in the clinical assessment are to be expected particularly in young and old persons (see also Fig. 1).
*Age \[years\]**Potentially over-diagnosed**Potentially under-diagnosed* !worddav5254ee43caf3dd1406afeeb83ee2a96a.png|height=360,width=634!
*Figure 1:*  *Comparison between diagnosing obstruction by using a fix FEV1/FVC ratio (blue line) and the use of an age-adjusted lower limit* *of normal (LLN, red line).* _Source: Mannino et al. 2007_
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The results of the breath test are displayed as flow-volume curve. The advantage of this view is that the patient's cooperation can be evaluated immediately and it becomes immediately evident if there is a ventilation disorder. In order to improve comparability of several measurements carried out with the same patient for example, the American Thoracic Society has requested a precisely defined form for the flow-volume curve, the so-called *2:1 view{*}.  Here, 2 liters are shown on the y-axis and 1 liter on the x-axis. This 2:1 view can be selected in custo diagnostic and be printed (see Fig.7).
!worddav6a342cc3c71523a4bcdda30917a4d5a7.png|height=336,width=490!
*Flow \[l/sec\]*
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*Figure 7: Flow-volume chart according to ATS (2:1 view)* 
A further specification according to ATS is that the expiratory *volume-time diagram* is displayed *over a period of 6 seconds* (see Fig. 8). This specification is also implemented in custo diagnostic.
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*Figure 8: Volume-time diagram only expiratory*
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For *quality management{*}, ATS requests that the date, the calibration result and the person having performed the calibration are protocolled each time a spirometer (Miller et al., 2005) is calibrated. This information is recorded and saved in custo diagnostic, together with the volume of the calibration pump, and can be called up and printed under the option "Calibrations" at any time.
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