Quantifying severity of airway obstruction
The European Respiratory Society, British Thoracic Society and American Thoracic Society (see international recommendations) have recommended to use the FEV1%(F)VC to decide whether there is or is not airway obstruction, and to quantify its severity on the basis of the FEV1. Therefore these recommendations have been adopted in the SpirXpert software. However, do keep in mind that using the FEV1 to grade severity of airways obstruction is fraught with difficulties:
- The recommendations relate to COPD and do not take account of coexisting restrictive ventilatory defects. Restrictive lung disease associated with an FEV1%(F)VC below a borderline leads to overestimating the severity of airway obstruction.
- Each assessment is very sensitive to how well the reference values adopted fit the population. In a large Dutch population, for example, adjustments of predicted values were required for FEV1 and FVC, but not for FEV1%FVC. The latter index has a strong point in its favor: even though predicted FEV1 and (F)VC differ widely in various parts of the world, differences in the level of these indices level out in the ratio, leading to remarkable international uniformity in the level of this index.
- In spite of ethnic differences in FEV1 and (F)VC, in western societies the FEV1%(F)VC differs little, if at all, between Caucasians and other ethnic groups, from childhood to old age. Publications from non-western countries seem to suggest that particularly in black women the FEV1/FVC ratio tends to be higher than in Caucasian women (see ethnic differences). With the potential exception of some non-western countries, therefore, FEV1%FVC offers a robust solution to diagnosing airway obstruction independent of ethnic differences in ventilatory function, but this is not the case for assessing its severity from the level of FEV11, in particluar because of the paucity of predicted values for non-Caucasians.
- General practitioners tend to underestimate FEV1 by up to 280 mL, depending on equipment used and previous training (see FEV1 underestimated). In general this is to be expected if spirometric measurements are not performed by professionally trained personnel and do not conform to international recommendations. If FEV1%(F)VC is below ‘normal limits’, airway obstruction tends to be systematically overestimated when based on the level of FEV1.
In keeping with international recommendations the severity of airway obstruction is based on the level of FEV1%predicted. The SpirXpert scaling complies with the BTS classification:
| Obstruction | FEV1%FVC | FEV1%predicted |
| None | > LLN | |
| Mild | < LLN | > 60% |
| Moderate | < LLN | 40-59 % |
| Severe | < LLN | < 40% |
| International recommendations | |
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| 3 | BTS. COPD guidelines. Thorax 1997; 52 suppl. 5: S1-S28. |
| 4 | National Institute for Clinical Excellence. Clinical Guideline 12. Chronic obstructive pulmonary disease, February 2004. (www.nice.org.uk/CG012NICEguideline). |
| 5 | ATS/ERS Task Force: Standards for the diagnosis and treatment of patients with COPD: a summary of the ATS/ERS position paper. Eur Respir J 2004; 23: 932–946. |
| 6 | Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Series "ATS/ERS Task Force: Standardisation of Lung Function Testing". Eur Respir J 2005; 26: 948-968. |
| Ethnic differences in FEV1%FVC | |
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| FEV1 underestimated | |
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