Positive AFB Sputum but Negative PPD Mantoux Test? How is this possible

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    Is it possible to have a positive acid fast bacilli sputum test and at the same time also have a negative mantoux ppd test?
    Both of these test for TB...so would having a + AFB test automatically mean that you have TB? I had a pt today who was growing bacteria in the sputum samples but they had a ppd test done and it was negative. I don't get it! Wouldnt this mean that you have active TB?
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  4. 8 Comments so far...

  5. 0
    I think it is possible to have a positive AFB and negative PPD because many things can be a factor such as immunosuppresion and all that, the person most likely has active TB and needs a chest x-ray for confirmation..
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    So a positive AFB test would be 100% that the person is infected with TB? Theres no way to have a positive cx and no evidence of TB right? Sorry for all the Q's I just find the diagnosis and tx of tb very difficult to understand sometimes. We didnt really go over it much even in community nursing class really...
  7. 1
    ppd, if administered correctly, will always be positive if the patient has been exposed to tb. it can, however, be incorrectly administered or read.
    acid fast + from septum can be caused from any organism from the mycobacteria family. however it is usually tb.

    odds would favor the initial ppd test being read or administered incorrectly. look for horses, not zebras. mycobacteria in septum that isn't tb is a zebra.
    MassED likes this.
  8. 2
    There are several reasons for a false negative Mantoux test.

    Severely weakened immune system: AIDS with low CD4 T cell count, corticosteroid or chemotherapy drugs,

    Vaccination with a live virus

    Improper testing: Sometimes the PPD tuberculin may be injected too deeply below the surface of the skin.

    Recent TB infection: It usually takes eight to 10 weeks after you've been infected for your body to react to a skin test.

    Overwhelming TB disease.
    MassED and NRSKarenRN like this.
  9. 0
    Quote from RN2B123
    So a positive AFB test would be 100% that the person is infected with TB? Theres no way to have a positive cx and no evidence of TB right? Sorry for all the Q's I just find the diagnosis and tx of tb very difficult to understand sometimes. We didnt really go over it much even in community nursing class really...

    Right, usually what's done is if its a + AFB then a chest x-ray is really needed for confirmation...As others have already stated, the PPD test has many possibilities for being negative such as an improperly done ID injection.. just as a positive PPD does not always mean active TB it could just mean exposure to the mycobacterium..
  10. 0
    Why bother administering a PPD if the sputum test was +?
  11. 1
    Quote from roser13
    Why bother administering a PPD if the sputum test was +?
    You usually will not know the results of the sputum sample right away. In fact, it might take 1 - 3 days to get the results. Some hospitals also still send out specimens to contracted labs.

    If you get a positve sputum test the first time, great. It is also typical to make 3 attempts to obtain a sputum over the course of 3 days. If the PPD is positive at the end of those 3 days and the clinical signs are significant even when the sputum is negative , a bronchoscopy may be needed or the tuberculosis might be somewhere other than in the lungs. A CXR is good indicator but if other infiltrates or lung disorders are present, it may make definitive interpretation difficult. Thus, other tests are done to confirm the presence of TB and of course, it doesn't have to be in the lungs.
    MassED likes this.
  12. 0
    I don't mean to hi-jack the thread but I have a couple of questions which are primarily for the LTC nurses although not exclusive. Do you utilize the 2-step PPD testing on admission to reduce false negatives and have you seen the booster phenomenon? Is this done for everyone or just the patients determined to be more at risk with immune system compromises other than just age?

    The LTC can also include those in the prisons and other long term incarcerations.
    Last edit by GreyGull on Sep 27, '10


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