BNP for AF - update
Systematic
review and meta-analysis of good studies of diagnostic tests are rare. One on
BNP tests for congestive heart failure is important as diagnosis is not easy in
primary care because symptoms are unspecific. Diagnosis usually requires
expensive echocardiography or radionuclide scan. BNP ruling out heart failure
would save money and time.
Systematic review
The
review [1] had a thorough search of general and specialist diagnostic test
databases. Studies compared any type of BNP assay in asymptomatic patients or
those with suspected congestive heart failure with gold standard of
echocardiography or radionuclide scan, with or without additional diagnostic
criteria, and have information on true and false positive and negative. Cut off
was that used by studies.
Results
Nineteen
studies on 22 populations with 9,093 patients were available. Studies were
generally of good quality, with prospective design, consecutive cohort, and
blind test interpretation of test results common. Mean ages ranged from 51 to
79 years, and percentage of men from 35% to 95%. Some studies examined
secondary care patients (acute dyspnoea, after myocardial infarction, with an
existing diagnosis of heart failure), others primary care referrals, and some
were screening studies of patients with risk factors.
The
results for ELISA and RIA methods are shown in Table 1 as the proportion of all
positive tests that were true positives and the proportion of all negative
tests that were true negatives. The ideal test would score 100% for both. The
proportion of patients who actually had heart failure by gold standard method
varied from 50-60% in the secondary care setting, to about 20% in patients
referred from general practice, and was 5% in screening studies. Where
prevalence was low, both types of tests reliably ruled out heart failure, so
that a negative test meant patients did not have heart failure.
Table 1: Results of BNP tests by ELISA or RIA in different populations with varying proportions of patients with true heart failure defined by gold standard diagnostic methods
| Population | (%) |
all positive (%) |
all negative (%) |
(%) |
all positive (%) |
all negative (%) |
||
| All studies | ||||||||
| Secondary care | ||||||||
| Primary care | ||||||||
| Screening | ||||||||
Comment
These
tests are not cheap, but the gold standard diagnostic test is much more
expensive. In a primary care population of 1,000 people in whom the GP has a
clinical suspicion of heart failure, 200 will actually have heart failure and
800 will not. Figure 1 shows how the test will work out if we assume that in
this population it picks up 50% of true positives and excludes 96% of true
negatives.
Figure 1: Results in a hypothetical primary care population of 1,000 people where GP suspects heart failure
Of
the 1,000 patients, the test would mean that 868 would not be sent for
confirmatory testing, while 132 would be sent for confirmatory testing. The
ratio of about 7 patients not sent for confirmatory testing because of the
result of the BNP testing for every one sent for confirmation would imply cost
saving if the confirmatory test cost about £100 or more, though it would
also have major implications for waiting times.
The
concern might be that 100 patients who truly had heart failure would not have
confirmatory testing. Presumably these would be less severe cases that might
return later to the GP, and would have other tests. More detailed thinking is
needed to fully appreciate the possible cost-effectiveness on BNP testing in
primary care.
Reference:
- M Battaglia et al. Accuracy of B-type natriuretic peptide tests to exclude congestive heart failure. Archives of Internal Medicine 2006 166: 1073-1080.
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