A very strange spirometry report came across my desk a couple of days ago.
| Observed: | Predicted: | %Predicted: | |
| FVC: | 3.07 | 0 | 29767 |
| FEV1: | 2.15 | 0 | 37586 |
| FEV1/FVC: | 70 | 71 | 101% |
My first thought was that some of the demographics information had been entered incorrectly but when I checked the patient’s age, height, gender and race all were present, all were reasonably within the normal range for human beings in general and more importantly, all agreed with what was in the hospital’s database for the patient. I tried changing the patient’s height, age, race and gender to see if it would make a difference and although this made small changes in the percent predicted when I did this the predicteds were still zero.
Or were they? They actually couldn’t have been zero, regardless of what was showing up on the report, since the observed test values are divided by the predicted values and if the predicted were really zero, then we’d have gotten a “divide by zero” error, and that wasn’t happening. Instead the predicted values had to be very close to zero, but not actually zero, and the software was rounding the value down to zero for the report. Simple math showed me the predicted value for FVC was (very) approximately 0.0103 liters, but why was this happening?