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Performance Evaluation Programs for Determining HIV-1 Viral Loads, Testing for HIV p24 Antigen, and Identifying Mycobacterium tuberculosis Using Nucleic Acid Amplification Tests

CDC's Division of Laboratory Systems, Public Health Practice Program Office, is implementing three new laboratory performance evaluation (PE) programs. The first assesses the performance of laboratories that perform tests to determine the viral RNA copy number (viral load) in the blood of persons infected with human immunodeficiency virus type 1 (HIV-1); the second assesses the performance of laboratories that perform HIV p24 antigen (Ag) testing; and the third assesses the performance of laboratories that perform nucleic acid amplification (NAA) tests for Mycobacterium tuberculosis.

Results of viral load determinations are being used by physicians treating HIV-infected patients to make decisions regarding initiation of antiretroviral therapy and to determine whether current antiretroviral therapy is effective or whether changes in antiretroviral therapy should be implemented based on the amount of virus in the blood of HIV-infected patients (1-4). In June 1996, the Food and Drug Administration (FDA) approved a reverse-transcriptase polymerase chain reaction procedure for determining viral load to assess patient prognosis.

In August 1995, FDA recommended that all facilities licensed for performing blood collections perform p24 Ag testing in addition to the HIV-antibody screening of blood-bank donors. The addition of p24 Ag testing would decrease the window period (7-11 days) for detecting donors recently infected with HIV when compared with the current protocol for antibody testing (i.e., enzyme immunoassay screening followed by Western blot supplemental testing). In 1996, FDA approved two p24 Ag tests.

Traditional laboratory tests for tuberculosis may require several weeks to complete; NAA tests that amplify M. tuberculosis DNA or RNA can identify within 1 day the organism directly in patient specimens that are positive for acid-fast bacilli by microscopy (5,6). Two NAA tests for M. tuberculosis complex have been approved by FDA, and other tests have been submitted for review. CDC has published interim guidelines (7) for FDA-approved M. tuberculosis NAA test usage, interpretation, and limitations. Laboratories using the FDA-approved M. tuberculosis NAA testing methods are encouraged to enroll in this PE program for information on evolving practice and performance issues (8) associated with these tests. Depending on availability of program resources, laboratories that use NAA tests developed in-house (9) also may enroll.

In each program, PE samples will be sent to participating laboratories twice yearly. After laboratories have tested the samples and reported their results (which are treated as confidential) to CDC, reports will be prepared and sent to participants, including aggregate testing results reported by other laboratories that used identical or similar test kits to test identical PE samples. After comparing their results with the aggregate results, laboratories can determine whether problems exist in their testing practices and, if applicable, correct them.

As with all PE programs at CDC, these new programs are strictly voluntary and are free to enrolled laboratory participants. The PE programs are designed to 1) establish a database containing information about the physical characteristics and testing practices of laboratories that perform viral load and p24 Ag tests for HIV and NAA tests to detect M. tuberculosis, 2) evaluate testing quality and to delineate factors that lead to high-quality testing, 3) monitor changes in testing in these testing sites over time, and 4) identify problems that occur in testing to maintain or improve testing quality.

For laboratories interested in participating in the new PE programs for viral load and p24 Ag testing, or one of the existing PE programs for HIV-1 antibody testing, human T-lymphotropic virus types I and II antibody testing, and T-lymphocyte immunophenotyping by flow cytometry and alternate methods, an enrollment form and additional information are available by calling (770) 488-4366 or (770) 488-4147 or faxing (770) 488-7693. For laboratories interested in participating in the new PE program for M. tuberculosis NAA tests or the current PE program involving M. tuberculosis drug-susceptibility testing, an enrollment form and additional information are available by calling (770) 488-4674 or faxing (770) 488-7663.

Reported by: Div of Laboratory Systems, Public Health Practice Program Office, CDC.


  1. Ho DD, Neumann AU, Perelson AS, Chen W, Leonard JM, Markowitz M. Rapid turnover of plasma virions and CD4 lymphocytes in HIV-1 infection. Nature 1995;373:123-6.

  2. Wei X, Ghosh SK, Taylor ME, et al. Viral dynamics in human immunodeficiency virus type 1 infection. Nature 1995;373:117-22.

  3. Mellors JW, Rinaldo CR Jr, Gupta P, White RM, Todd JA, Kingsley LA. Prognosis in HIV-1 infection predicted by the quantity of virus in plasma. Science 1996;272:1167-70.

  4. Saag MS, Holodniy M, Kuritzkes DR, et al. HIV viral load markers in clinical practice. Nature Medicine 1996;2:625-9.

  5. Miller N, Hernandez SG, Cleary TJ. Evaluation of Gen-Probe amplified Mycobacterium tuberculosis direct test and PCR for direct detection of Mycobacterium tuberculosis in clinical specimens. J Clin Microbiol 1994;32:393-7.

  6. Vuorinen P, Miettinen A, Vuento R, Hallstr˝m O. Direct detection of Mycobacterium tuberculosis complex in respiratory specimens by Gen-Probe amplified Mycobacterium tuberculosis direct test and Roche Amplicor Mycobacterium tuberculosis test. J Clin Microbiol 1995;33:1856-9.

  7. CDC. Nucleic acid amplification tests for tuberculosis. MMWR 1996;45:950-2.

  8. Noordhoek GT, van Embden JDA, Kolk AHJ. Reliability of nucleic acid amplification for detection of Mycobacterium tuberculosis: an international collaborative quality control study among 30 laboratories. J Clin Microbiol 1996;34:2522-5.

  9. CDC. Diagnosis of tuberculosis by nucleic acid amplification methods applied to clinical specimens. MMWR 1993;42:686.

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