A couple of years ago, a clinical laboratory made use of the fact that canine heart disease leads to an increase in a product (called B-type natriuretic peptide, or BNP) within the blood stream. The Idexx Cardiopet pro-BNP test looked like a good way to measure and quantify heart disease simply by drawing a blood sample. The method was (and is) much less involved and expensive than echocardiography, which is the gold standard test for canine heart disease.
When I first heard about BNP, I was reminded of the prostate-specific antigen (PSA) test in humans. The PSA is used as a screening tool for prostate cancer. It turns out that the similarities between the tests are not limited to their respective uses, but also to the controversies surrounding their use.
The PSA test is designed around the fact that cancerous prostate tissue produces relatively large quantities of the antigen. However, normal prostate tissue also produces PSA. Therefore, although an elevated PSA measurement may mean that a man’s prostate is undergoing dangerous malignant changes, it also may mean nothing whatsoever. Some physicians (most notably Nortin Hadler, MD, who wrote eloquently about the matter in his heretical book, Worried Sick) have argued that in many cases the PSA test does more harm than good. It has led many men to suffer needless worry, not to mention invasive biopsies and prostatectomies (which often lead to incontinence and impotence).
The pro-BNP test certainly hasn’t led to this much controversy. However, a recent paper in the Journal of Veterinary Emergency and Critical Care has made a point that should be obvious when one considers that B-type is short for Brain-type: heart disease is not the only possible cause of elevated BNP levels. Here are the paper’s conclusions, with much of the medical jargon translated into English.
Conclusions — A clinically relevant proportion of [dogs with no breathing difficulties] with [conditions not related to heart disease] have increased cBNP concentrations that exceed previously identified diagnostic thresholds, potentially limiting the ability of this test to identify [congestive heart failure] when [other problems are present].
Does this mean that the pro-BNP test is useless? Certainly not. It does, however, mean that the test, like everything in medicine, must be used judiciously and the results must be interpreted in context.
The paper cited in this post is Lee et al, Journal of Veterinary Emergency and Critical Care 21(1) 2011, pp 5-12.