REVIEW OF RAPID DIAGNOSTIC TESTS FOR TUBERCULOSIS 

Submitted by Dr. Maricela Moffitt, Maricopa County Deputy TB Control Officer

Remember when we routinely waited several weeks for lab results to confirm a suspected diagnosis of tuberculosis? Now labs may be able to identify M. tuberculosis in as little as 24 hours. (OK, sometimes it still takes several weeks.) How do they do it? How reliable are the results?

Two methods can be used to identify mycobacteria directly from a sputum specimen. High-performance liquid chromatography (HPLC) recognizes several mycobacteria species based on the mycolic acid “fingerprint” pattern with great accuracy. However, it requires the presence of a fair number of organisms in the sputum. Our State Laboratory currently does HPLC on sputum AFB smears which are 2+ or greater.

The other method of direct identification from sputum is nucleic acid amplification (NAA). Two tests are currently available in the US, MTD-2 (by Genprobe) and Amplicor (by Roche). MTD-2 is a DNA amplification technique. Amplicor uses PCR amplification. Both are FDA-approved for smear positive sputum specimens. Only MTD-2 is approved for smear negative specimens where there is high clinical suspicion of tuberculosis. The CDC recommends that if the smear and NAA are both positive, the patient can be presumed to have TB. If the smear is positive and the NAA is negative, TB is not ruled out. The NAA may sometimes be falsely negative due to the presence of inhibitors in the sputum. In this situation, the CDC recommends sending another specimen for repeat NAA testing and a test for inhibitors. If the repeat smear is still positive, and the NAA is still negative, and no inhibitors are detected, the patient may be presumed to have non-tuberculous mycobacteria. Due to lower reliability in smear negative cases, MTD should only be run if there is a high index of suspicion. If two MTD tests are positive on separate smear-negative specimens, the patient can be presumed to have TB. However, if smears are negative and the MTD is negative, these results should not affect patient management. Up to 50% of these cases will ultimately prove to be culture positive for M. tb.

State Lab will be moving toward more use of HPLC for direct identification of mycobacteria in the future due to new equipment which should able to make an identification on smears with less than 2+ positivity. HPLC has at least two advantages over NAA. It is substantially cheaper (thus enabling testing of more specimens with the same public health buck). Also, it is able to specifically identify several species of mycobacteria, as opposed to NAA which can only tell you whether it’s TB or not TB. Both are considered preliminary test results, to be confirmed by the gold standard culture. Cultures continue to be needed to determine drug susceptibility.

“NAA tests can enhance certainty, but they do not replace AFB smear or mycobacterial culture, and they do not replace clinical judgment. Clinicians should interpret these tests based on the clinical situation, and laboratories should perform NAA testing only at the request of the physician and only on selected specimens.” (MMWR, vol. 49, no.26, July 7, 2000.)

The National Tuberculosis Controllers Association (NTCA) held its annual meeting in June in Alexandria, VA. Some interesting items presented include:

New TB control guidelines for hospitals are expected from the CDC in approximately spring of 2003. These will update “Preventing Transmission of M.tb in Health-care Facilities, 1994.” One significant change will be a simplification of the hospital risk assessment for TB, which determines how often employees must be skin tested. In the new guidelines there will be only 3 risk categories, low, medium, and ongoing transmission. Guidelines for discontinuing isolation will be a little different: isolation may be discontinued when 1) the likelihood of infectious TB is negligible, AND 2) either another diagnosis is made OR there are 3 negative sputum smears. The 3 sputa no longer have to be collected on different days, but must be at least 8 hours apart and one must be an early morning specimen. The new guidelines will extend to non-hospital settings such as correctional facilities, hospices, laboratories, and emergency medical services.

Soon-to-be released updated treatment guidelines from American Thoracic Society (ATS) and CDC for active TB were previewed. Standard therapy has not changed. However, an additional 3 months of treatment (7 months of continuation phase, total 9 months) will be recommended for patients who have cavitary disease and whose sputum cultures have not converted to negative by 2 months into treatment. Once weekly INH and rifapentine may be used in the continuation phase for selected patients (non-cavitary disease, HIV negative, must be smear negative at 2 months). Levofloxacin, moxifloxacin, and gatifloxacin may be used when there is resistance to first line drugs or the patient is unable to tolerate them.

TB research with CDC involvement was presented. We’ll review that in a coming issue of the TB newsletter.

Test kits for the new TB blood test, QuantiFERON-TB, are now available through the new U.S. headquarters of Cellestis, Inc. (See the March newsletter for more info about the test-if you didn’t get it and want a copy, please email Cheryl McRill.) Please note that this is a moderately complex laboratory test which must be performed by a CLIA certified lab. It is not an office procedure. As of this moment, there are no commercial, hospital, or public health labs offering this test to patients, but several local labs are exploring its use. Contact info for interested labs: Cellestis, Inc., 28005 N. Smyth Drive, Valencia CA 91355, toll free 800-519-4627, fax 661-295-4625.

CDC guidelines for use of the QuantiFERON-TB blood test are expected very soon. The current draft of the guidelines was reviewed at the recent National TB Controllers Association meeting. There will be two cut-offs for positive results, one for patients at low risk for latent TB infection and one for those at high risk. A point of discussion which is apparently still undecided is whether to recommend confirmation of positive results with a traditional TB skin test before beginning treatment for latent infection.

The State Lab is now including ofloxacin as part of its routine plate drug susceptibility panel for M. tuberculosis. While ofloxacin is not the best clinical choice for TB among the quinolones, it is cross-resistant or cross-susceptible with the other quinolones. In other words, the isolate may be presumed to have the same susceptibility or resistance to levofloxacin or ciprofloxacin as it has to ofloxacin. Kanamycin was deleted from the panel to make room for ofloxacin, but is still available on request if needed. As in the past, most specimens will receive only susceptibility testing to the first line drugs (isoniazid, rifampin, pyrazinamide, ethambutol, and streptomycin). These are done by the broth culture method which is much faster than the plate method used for the full panel. The plate panel will always be done automatically whenever there is resistance found to any first line drugs other than isoniazid.

The State Lab will soon be going to seven day per week staffing. The full crew will not be there on Saturday and Sunday, but the weekend staff will be able to receive sputum specimens and do AFB smears. This is really important because the lack of weekend services is a reason some hospitals have given for not using State Lab for TB. The State Lab offers top-notch knowledge and experience, and state-of-the-art techniques. Plus everything is done in house, not shipped out of state. So if your local hospitals are not using the State Lab for TB, please let them know they will soon be able to get this excellent service seven days per week, and it’s still free!! Don’t send those weekend specimens yet, but watch for the announcement soon.

Please direct any comments, questions, or items you would like to submit for the newsletter to: Cheryl McRill, M.D., M.P.H. State TB Control Officer Arizona Department of Health Services 150 N. 18th Ave, Suite 140, Phoenix, AZ 85007 cmcrill@azdhs.gov 602-364-3856.