Well, not necessarily anything, although as usual that depends on the circumstances. Recently I was contacted by an individual who was concerned that their DLCO had decreased from 120% of predicted to 99% of predicted. They also mentioned that their DLCO results have normally ranged from 117% to 140% of predicted over the last 9 months.
More interestingly however, they said that
“the technician told me before I even took the test that anything over 100% for DLCO is essentially a testing error.”
Wow. That statement is wrong on so many levels it’s hard to know where to start but I’ll give it a shot anyway.
First, there are a variety of DLCO reference equations. The ATS/ERS guidelines recommends that PFT Labs pick the reference values that most closely matches their patient population but how this is done is left to individual labs. There are at least a couple dozen DLCO reference equations to choose from and probably about a half dozen of these are in common use in PFT labs around the world.
Because no patient population is ever going to precisely match those of a study this means that DLCO results are going to tend to be above or below 100% of predicted depending on which reference equation the lab is actually using. This also means that if results from otherwise normal subjects are mostly above or mostly below 100% of predicted then the wrong reference equations are being used.
Obesity has become far more commonplace than it was a generation ago. The reasons for this are unclear and have been attributed at one time or another to hormone-mimicking chemicals in our environment, altered gut biomes, sedentary lifestyles or the easy availability of high calorie foods. Whatever the cause, obesity affects lung function through a variety of mechanisms although not always in a predictable manner.
Many investigators have shown a relatively linear relationship between an increase in BMI and decreases in FVC and FEV1. These decreases are small however, and FVC and FEV1 tend to remain within normal limits even in extreme obesity. The decreases in FEV1 and FVC tend to be symmetrical which is shown by the fact that the FEV1/FVC ratio is usually preserved in obese subjects without lung disease. Several studies have shown that the decreases in FVC and FEV1 are reversible since a decrease in weight showed a corresponding increase in FVC and FEV1.
In one study a 1 kg increase in weight correlated with a decrease in FEV1 of approximately 13 ml in males and 5 ml in females. The same increase in weight correlated with a decrease in FVC of approximately 21 ml in males and 6.5 ml in females. The greater change in FVC and FEV1 in males than females has been attributed to the fact that males tend to accumulate extra weight primarily in the abdomen.
The notion that abdominal weight has a disproportionate effect on lung function is seconded to some extent by studies that have shown that decreases in FVC and FEV1 correlated better with increases in waist circumference and the waist to hip ratio than with BMI. One study showed a 1 cm increase in waist circumference caused a 13 ml reduction in FVC and an 11 ml reduction in FEV1 across a range of elevated BMI’s.