An evidence based analysis of the commonly ordered and frequently misunderstood lab test: BNP. First post in a new series titled: “Night School” at the Old Boston City Hospital, by emergentologist, educator and nocturnist: Dr. Steve McGuire.

There are only a few things I know about the world. Sunrise in the east, sunset in the west. Death and taxes. Richard Simmons is being held hostage by his housekeeper. In the ED, there are more questions and fewer certainties.

“Do you want me to order BNP?”

Shortness of breath is a common (3+ million visits/yr) presenting ED complaint in the US(1). Acute decompensated heart failure is only one of the many diagnoses that should be considered for the patient with undifferentiated SOB. So then, how can we be certain if we should treat a patient for heart failure.

The BNP question might seem appealing(2,3), especially for those of us that have not been practicing for 40 years, and who would still like to ‘get it right’ when it comes to diagnosing our patients with difficulty breathing. And it is a common part of the workup seen in ED’s across the country. But can BNP really help us differentiate among the many overlapping disease states that can cause shortness of breath?

Martindale and colleagues reviewed the literature and showed that among other things, using BNP to rule out ADHF is, at least, a thing. A result of BNP below some low threshold was dispositive. For example, a value <100pg/ml generally had sensitivities >93% depending on the assay used. But there is danger of using a continuous variable like BNP as a ‘switch’, or binomial tool. This is demonstrated by the drastic change in sensitivity for even a minor increase in cutoff level, e.g. 200pg/ml dropped sensitivity to 85%, 500pg/ml down to 68%(4). At what level can you be reasonably sure the symptoms are from BNP?

Multiple studies show that good clinical judgement is at least as predictive as any BNP biomarker testing. McCullough showed that when the ED doc (you) was “sure” (>80% certain of ADHF) they performed better than BNP at a cutoff of 100pg/ml (specificity 96% v. 73%)(5).  Put another way, you can ‘rule in’ ADHF better than the test.  Similarly, when the ED doc was “sure” that the symptoms were not from CHF, BNP testing added little to nothing. It is also worth noting as discussed by Rory Spiegel that these results were dubiously compared to the judgement of 2 cardiologists who reviewed medical records.

“But what about if we’re not sure?” Now you might be asking the right question. The problem is that BNP doesn’t test for CHF. There are so many states, even some non-pathological, that elevate BNP levels, that a BNP level is neither sensitive nor specific enough to hang your hat on(6). And there are so many other variables that are at least as predictive (7). You have better tools in your tool belt.

“But they trend it on the floors.” That’s not really a good reason either. Wu showed that biologic variability made daily testing of dubious value(8). Essentially, changes of up to 40% can be ascribed to biologic variability. Additionally you can remind your inpatient colleagues, that they have judgment that is again the equal of the data supporting the industry sponsored studies that promote BNP as the answer. Clinically significant outcomes were not changed when BNP managed care was compared to clinical judgment in these studies by Luc(9), Persson (10), and Kalström (11). Nor was BNP in the ED likely to be helpful as  reviewed by Carpenter, Rosen and others(12).

BNP is an answer looking for a question.

You can read more from the perspective of an EM resident Mike Lamberta @EMLamberta here.

References:

  1. Maisel AS, Krishnaswamy P, Nowak RM, et al. Rapid measurement of B-type natriuretic peptide in the emergency diagnosis of heart failure. N Engl J Med. 2002;347:161-167.
  2. Rui P, Kang K, Albert M. National Hospital Ambulatory Medical Care Survey: 2013 Emergency Department Summary Tables. Available from: http://www.cdc.gov/nchs/data/ahcd/nhamcs_emergency/2013_ed_web_tables.pdf.
  3. Circulation. 2013;127:500-508
  4. Martindale, J.L., Wakai, A., Collins, S.P., Levy, P.D., Diercks, D., Hiestand, B.C., Fermann, G.J., deSouza, I., Sinert, R. Diagnosing Acute Heart Failure in the Emergency Department: A Systematic Review and Meta-analysis. Acad Emerg Med. 2016;23:223–242.
  5. McCullough PA, Nowak RM, McCord J, et al. B-type natriuretic peptide and clinical judgment in emergency diagnosis of heart failure: analysis from Breathing Not Properly (BNP) Multinational Study. Circulation. 2002;106:(4)416-22
  6. Schwam, E. (2004), B-type Natriuretic Peptide for Diagnosis of Heart Failure in Emergency Department Patients: A Critical Appraisal. Academic Emergency Medicine, 11: 686–691. doi:10.1197/j.aem.2003.12.024
  7. Wang CS, FitzGerald JM, Schulzer M, Mak E, Ayas NT. Does this dyspneic patient in the emergency department have congestive heart failure? JAMA 2005;294:1944–56
  8. Wu AHB. Serial testing of B-type natriuretic peptide and NTpro-BNP for monitoring therapy of heart failure: the role of biologic variation in the interpretation of results. Am Heart J. 2006;152(5):828-834.
  9. Journal of the American College of Cardiology Volume 56, Issue 25, 14–21 December 2010, Pages 2090–2100
  10. European Journal of Heart Failure, 12: 1300–1308
  11. European Journal of Heart Failure, 13: 1096–1103
  12. Carpenter CR, Keim SM, Worster A, Rosen P, BEEM (Best Evidence in Emergency Medicine). BRAIN NATRIURETIC PEPTIDE IN THE EVALUATION OF EMERGENCY DEPARTMENT DYSPNEA: IS THERE A ROLE? The Journal of Emergency Medicine. 2012;42(2):197-205.

 

Faculty Author: Steve McGuire, DO