Background: The AMPLICOR assay (Roche; Branchburg, NJ),
a rapid direct amplification test for Mycobacterium
tuberculosis, has only been licensed for use in smear-positive
respiratory specimens. However, many patients with pulmonary
tuberculosis (PTB) have smear-negative disease. The clinical utility of
this test in patients with smear-negative PTB is unknown.
Objective: To evaluate the effect of pretest probability of
PTB estimated by chest physicians on the accuracy of the AMPLICOR assay
in patients with smear-negative PTB.
methods: A prospective study of consecutive patients suspected of
having smear-negative PTB. Two chest physicians estimated the pretest
probability of active disease (high, intermediate, and low categories).
Respiratory specimens were examined with radiometric broth medium
cultures and with the AMPLICOR assay for M
tuberculosis. The decision on a final diagnosis of PTB was
blinded to the AMPLICOR results.
Results: Active PTB
was diagnosed in 25 of 441 patients (5.7%). The AMPLICOR assay had an
overall sensitivity of 44% and a specificity of 99%. Results of the
assay were negative in seven patients with culture-negative PTB. The
proportions of patients in the high, intermediate, and low pretest
groups were 4.5%, 19.7%, and 75.7%, respectively. The incidence of
PTB for each group was 95%, 3.4%, and 0.9%, respectively. The
sensitivities of the AMPLICOR assay in the three groups of patients
were 47%, 33%, and 33%, respectively, while the specificities were
100%, 98%, and 99%, respectively.
patients suspected of having smear-negative PTB, the following
conclusions were drawn: (1) the incidence of active PTB was low; (2)
pretest estimates accurately discriminated between patients with high
and low risk of PTB; (3) the risk of PTB was overestimated in the
intermediate group; and (4) the utility of the AMPLICOR assay in the
intermediate-risk group may be limited by the overestimation of disease
prevalence and low test sensitivity. Further studies are needed on the
role of the AMPLICOR assay in better selected patients with an
intermediate risk of having smear-negative PTB.