RESEARCH - Blood tests may be better than skin tests for diagnosing TB

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Blood tests may be better than skin tests for diagnosing TB

Rheumawire

Apr 25, 2006

Gandey

Modena, Italy - New blood tests produce fewer false-positive results than

the standard tuberculin skin tests, researchers report. " We finally have a

replacement for the oldest TB test in the world, " senior author Dr Luca

Richeldi (University of Modena and Reggio Emilia, Italy) told rheumawire.

" These two new commercially available blood tests are more specific than

skin tests. " But he added that his group, reporting in the April 22, 2006

issue of the Lancet, also found that results differed between the two blood

tests-especially in immunosuppressed patients and young children [1]. These

findings suggest that the new tests might produce discordant results in

routine clinical practice.

Despite the limitations of the tests, Richeldi predicts that these new blood

tests will become a gold standard. " The skin test is the current standard of

care, but it has never been considered a gold standard because it lacks

sensitivity. In my opinion, the blood tests have this potential because they

are more accurate. "

Richeldi adds that the two tests-T-SPOT.TB (Oxford Immunotec, Abingdon, UK)

and QuantiFERON-TB Gold (Cellestis, Carnegie, Australia) also provide

operational advantages over the tuberculin skin test. No follow-up visit is

required, results are available within 24 hours, and a boosting effect is

not a concern with repeated testing.

The present analysis was supported by the Azienda Ospedaliera Policlinico di

Modena in Italy, and the testing kits were provided by the manufacturers of

the blood tests. T-SPOT.TB has been approved for in vitro diagnostic use in

Europe and is currently being assessed by the US Food and Drug

Administration. QuantiFERON-TB Gold is approved in the US for in vitro

diagnostic use.

Fewer false positives with blood tests on vaccinated patients

In this prospective study, the investigators, led by Dr Giovanni Ferrara

(University of Modena and Reggio Emilia), looked at more than 390

consecutively enrolled patients with suspected latent or active

tuberculosis. Patients were tested, and the performance of each blood test

was compared with the skin test.

The researchers found that fewer BCG-vaccinated individuals were identified

as positive by the two blood assays than by the tuberculin skin test

(p=0.003 for T-SPOT.TB and p<0.0001 for QuantiFERON-TB Gold).

But Ferrara and his team report that the blood tests were affected by

factors potentially associated with reduced functioning of the cellular

immune system such as age or immunosuppressive treatments.

" In children aged five years or younger, T-SPOT.TB performed better than

QuantiFERON-TB Gold, which might be clinically relevant since young children

with recent primary infection are at increased risk of progressing to

tuberculosis and can test as false-negative with the tuberculin skin test, "

they note. Systemic steroids had a similar though nonsignificant effect on

the accuracy of QuantiFERON-TB Gold and T-SPOT.TB.

" It is noteworthy, however, that there are differences between the two blood

assays when they are used in routine practice-especially in the diagnosis of

high-risk individuals, " Ferrara and colleagues write. " The choice of which

diagnostic test to use should depend on the population being tested, the

purpose of testing, and the resources available. "

Guidelines on the use of QuantiFERON-TB Gold have been released by the US

Centers for Disease Control and Prevention, and both tests are included in

the UK guidelines on tuberculosis published by the National Collaborating

Centre for Chronic Conditions.

Source

1. Ferrara G, Losi M, D'Amico R, et al. Use in routine

clinical practice of two commercial blood tests for diagnosis of infection

with Mycobacterium tuberculosis: A prospective study. Lancet 2006;

367:1328-1334.

Not an MD

I'll tell you where to go!

Mayo Clinic in Rochester

http://www.mayoclinic.org/rochester

s Hopkins Medicine

http://www.hopkinsmedicine.org