fixed ratio vs LLN for spirometry

There is ongoing and vigorous discussion, and sometimes confusion, regarding how to appropriately diagnose airflow obstruction, the major physiological feature of COPD, and this discord remains a major hurdle to improving management of patients with COPD.  Recent results examining a large population-based cohort followed for more than 15 years has found that defining airflow obstruction as FEV1/FVC <0.7 provides discrimination of COPD-related hospitalization and mortality that is more accurate than using the LLN. 

Major international respiratory society guidelines recommend diagnosing COPD using the fixed threshold FEV1/FVC <0.7, an approach analogous to current clinical practice in hypertension and diabetes, for which the identification of fixed disease thresholds has led to improvements in early detection and treatment. However, this historical determination was largely set by expert opinion and not well confirmed. An alternative approach is to define COPD with the FEV1/FVC LLN based on predicted values derived from population-based normative data adjusted for age, sex, race, and height. Concern regarding the LLN includes potential underdiagnosis in older individuals, and overdiagnosis in younger individuals. Similarly, concern regarding use of the fixed threshold FEV1/FVC <0.7 includes potential underdiagnosis of COPD in younger individuals and overdiagnosis of COPD in older individuals. However, regarding possible overdiagnosis by fixed thresholds, prior literature has established that up to 20% of older adults who fulfill the 0.70 threshold, but not the LLN criteria, have a greater degree of structural lung disease on CT scan, worse quality of life, and greater health care utilization and mortality when compared with individuals without airflow obstruction by either criterion. 

Compared with the fixed threshold which is less specific than the LLN, use of the LLN is less sensitive and fails to identify a number of patients with significant pulmonary pathology and respiratory morbidity. Other FEV1/FVC fixed thresholds ranging from 0.40 to 0.80 have been compared, revealing that 0.70 and 0.71 have the highest, and not significantly different from each other, discriminatory accuracy with respect to COPD hospitalization and mortality. In summary, FEV1/FVC <0.7 performs better than the FEV1/FVC less than LLN for discriminative accuracy of COPD-related hospitalization and mortality.

It is however pragmatic to acknowledge that neither the fixed threshold <0.7 or the LLN are perfect, inferring that spirometry alone defined COPD does not detect all individuals at risk of respiratory related events. It is also essential to recognize that there are individuals for which the diagnosis is unclear because of a borderline FEV1/FVC value, particularly in the absence of symptoms. In these instances, it is clinically appropriate to continue to monitor the patient and/or undertake additional investigation during which time the diagnosis may become more evident.1

Footnotes

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