Error in measuring airflow obstruction, fev1/fvc ratio
I have noticed that in several places where I work(NHS,industry)as an occupational health physician,there appears to be an anomaly with the automatically calculated fev/fvc ratio.
It would appear that all the machines on occasion will calculate fev/vc instead resulting in a false positive screening pick-up of obstructive patterns .
trying to get any sense out of vitalograph has bben non-productive,although one engineer did say that the machines were programmed to take the ratio using whichever is the largesti.e. fvc orvc.
surely this misses the point for picking up the early dip in forced manoevres which is characteristic of obstructive pattern?
ANY IDEAS?
Confused,north wales.
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There are 3 ways of expressing an index of airflow obstruction from spirometric manoeuvres. In England we mostly use the FEV1/FVC, as a slow vital capacity (VC) is often not measured to reproducible standards in surveillance spirometry. In patients with severe airflow obstruction the slow VC is often larger than the forced vital capacity (FVC). In near normal workers at surveillance examinations the differences are likely to be too small to worry about, measuring FVC alone I believe is adequate. If you were in France you would measure the FEV1/inspired vital capacity (FEV1/IVC), the Tiffeneau index. The interpretation of all of these is difficult in healthy workers, as there are a group with normal (or high) values for FEV1, supranormal values for vital capacity, and reduced ratios. Such results are sometimes found in the supernormal, such as elite athletes, divers etc.
For surveillance I believe that the FEV1 measurement to approved standards of acceptability and reproducibility is difficult enough to achieve. Interpretation of results is even more difficult. The most helpful interpretation uses the worker as their own control, looking at changes over time. Even this is difficult as changes up to 400ml can occur for no obvious reason between two visits (this is equivalent to about 12 years normal deterioration). If this occurs in an asymptomatic worker it should be repeated in a month or two, and will usually be back to baseline values.
Surveillance for most airways diseases is usually best done with a questionnaire, and investigation including serial peak flow measurements made for those with new work-related symptoms. There are however some workers with significant asthma who deny symptoms, these should be picked up by spirometry.
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Please accept our apologies if a confused answer was given - however the response received was the correct one, but perhaps would have been clearer with an explanation as to the reason.
Vitalograph Spirometers meet the accepted standards for Spirometry, and are programmed to allow clinicians to carry out testing as per these guidelines - the current European Respiratory Society Guidelines on Standardized Lung Function Testing (Eur Respir J, 1993, 6, suppl 16 - sec 3.4.9) stated that the largest value of VC, IVC or FVC should be reported (and therefore used to calculate the FEV1 ratio).
Measurement of both the VC and FVC will frequently produce higher slow Vital Capacities than Forced ones, due to Airways trapping, difficulty experienced in patients exhaling fully in the forced manouevre, early test termination etc. If both tests have been measured correctly, and are reproducible within the limits set by the guidelines then it is appropriate that the higher value be used, and therefore the low ratio may not be a false positive, but an indication that there is something wrong (if only poor patient technique!)
Please also note the response from Dr Sherwood Burge.
I trust this helps
Marcus Garbe, Vitalograph Ltd
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