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    Home > Active Ingredient News > Infection > False positive for acid-fast bacilli?

    False positive for acid-fast bacilli?

    • Last Update: 2021-04-21
    • Source: Internet
    • Author: User
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    It is only for medical professionals to read for reference.
    What clinical medicine needs is a comprehensive analysis of the condition in order to get a more accurate diagnosis, and it cannot be completely dependent on the laboratory.

    Foreword The diagnosis of tuberculosis, the experience of clinicians is very important! However, laboratory inspections are also very important.

    If a clinician diagnoses tuberculosis, there are very few positive for acid-fast bacilli, it may be that you have misdiagnosed too much, or the technical level of the laboratory has a lot of room for improvement.

    Because as early as 2010, the fifth national tuberculosis epidemiological survey found that China had 4.
    99 million active tuberculosis and 1.
    29 million bacteria-positive tuberculosis (positive sputum smear or positive sputum culture), 129/499=25.
    85% .

    In other words, the average level in China, about 25% of tuberculosis can be detected with acid-fast bacilli or tuberculosis bacterium cultured! Are you holding back? A positive acid-fast bacillus is not necessarily tuberculosis, because there are three possibilities for a positive acid-fast bacillus: 1.
    Tuberculosis (the most likely); 2.
    Other bacterial infections; 3.
    False positives.

    Let me share with you a case from Peking Union Medical College Hospital.
    Through this case, we will have a more profound experience and understanding of anti-acid bacteria positive.

    The case introduced an elderly man with fever and skin rash for 3 months.

    Before 3 months, the patient had repeated moderate fever at night with dry cough.
    Penicillin treatment by the local hospital was ineffective.
    Scattered red macules appeared on the skin all over the body without obvious itching.

    Chest CT showed inflammation in the posterior basal segment of the left lower lung (Figure 1).
    Penicillin and ceftriaxone were continued to be given to fight infection (this operation is very strange).
    The fever did not relieve, and the rash gradually increased.
    Reexamination of the left lower lung lesions increased, and lung biopsy was performed.
    , The pathology was chronic granulomatous inflammation with more plasma cell infiltration, and the skin biopsy pathology was tuberculosis-like nodules; the sputum smear was positive for acid-fast bacilli, diagnosed as "tuberculosis", transferred to a hospital in Beijing, and was given isoniazid and rifone Pentin and other anti-tuberculosis treatments.

    Figure 1 However, anti-tuberculosis was ineffective for half a month.
    A lung biopsy was performed again.
    Pathology showed that a large number of cryptococcus was found in the necrotic tissue.
    PAS staining (+) was transferred to Peking Union Medical College Hospital for further diagnosis and treatment.

    What kind of disease do you think the patient is? tuberculosis? Non-tuberculous mycobacteria infection? Cryptococcosis? Interstitial pneumonia? Tumor? Physical examination during the diagnosis and treatment of Peking Union Medical College Hospital: purple-red papules and plaques were seen on the head, face, back, upper arms and both thighs, and some of the surfaces were scabs.
    The rest of the physical examination showed no abnormalities.

    Laboratory examination: white blood cells are slightly elevated, mainly neutral.

    PPD (+).

    Liver and kidney function and tumor markers were normal.

    Cerebrospinal fluid is routine and biochemically normal, serum and cerebrospinal fluid cryptococcal antigen (-), cerebrospinal fluid ink staining to find Cryptococcus (-).

    Glucose tolerance test: 2 hours after oral glucose, the blood sugar was 17mmol/L, and he was diagnosed as type 2 diabetes.

    Auxiliary examination: lung function showed mild obstructive ventilatory dysfunction, with a slight decrease in diffusion function.

    First, amphotericin B and fluconazole were given according to disseminated cryptococcosis, and the evidence of cryptococcal infection was continued.

    Local puncture smears of the skin lesions showed yeast-like bacteria.

    New skin lesions were re-skin biopsy for pathology and fungal culture.

    The culture time was extended to 21 days, and the same biphasic fungus was isolated from 4 sputum specimens and skin lesions, which was finally identified as dermatitis blastomycetes.

    He continued to use amphotericin B with a cumulative dose of 5.
    359 grams.
    He was hospitalized for 8 months, and his condition improved and was discharged.
    The oral itraconazole was continued outside the hospital.

    Why is this patient positive for acid-fast bacilli? First of all, it is not tuberculosis.

    Secondly, non-tuberculous mycobacteria, leprosy bacilli, etc.
    , are also positive for acid-fastness.

    Nocardia also has weak acid resistance.

    It has been reported in the literature that some coryneform bacteria and actinomycetes also have weak acid resistance; a very small number of Propionibacterium greedy may also be positive for acid resistance.

    However, this patient was finally diagnosed with pulmonary fungal disease, and the above-mentioned special bacterial infection could not be used to explain the positive acid-fast bacilli.

    Then, the most likely thing is: false positive for acid-fast bacilli! It is to mistake some positive bacteria, positive bacteria particles, and false positive foreign bodies on the slides as positive acid-fast bacilli.

    Why is there a false positive for acid-fast bacilli? Positive bacilli: As shown in the figure below, it is acid-fast positive at first glance (A), but when the micro-spiral is rotated slightly in the non-focused state, the bacteria are blurred and non-refractive (B).

    Figure 2 Positive bacteria particles: It is the disintegration product of positive bacilli, which is fine, red stained, non-refractive (very important), with varying degrees of singleness scattered or mixed between positive bacilli (Figure ② below), BCG immunity It is brownish-yellow to tan during histochemical staining (Figure 4 below).

    Figure 3 False positive foreign body: The fine foreign body deposited in the tissue section by the dye solution can easily be mistaken for positive bacteria with acid-fast staining.

    The difference with acid-fast staining positive bacilli is: ①The most important thing is that when the micro spiral is slightly rotated, it appears on the surface of the tissue section (that is, it does not exist at the same imaging level as the tissue section), and shows a colorless, shiny refractive index ② Irregular shape, inconsistent length and thickness; ③ How light and bright red staining; ④ In the tissue section, it can be distributed in areas unrelated to the disease.

    In the figure below, A and B are the same field of view.
    When the micro spiral is rotated slightly, bright red-stained foreign bodies appear on the surface of the tissue section, that is, they do not exist at the same imaging level as the tissue section, and a colorless, shiny refraction is seen.

    Figure 4 Postscript Clinical medicine needs a comprehensive analysis of the condition to get a more accurate diagnosis, and it cannot rely entirely on laboratories.

    To eliminate the false positive foreign body caused by glutinous acid-fast acid staining, 3 points need to be done: The most important thing is to cover a white tissue sheet slightly larger than the tissue white sheet and soaked with pure distilled water on the white tissue sheet prepared for dyeing with carbolic acid redness.
    Filter paper, drop the carbolic acid reddening liquid on the surface of the filter paper, and then dye it under the heating of an alcohol lamp; secondly, there should be no sediment in the reduping liquid; finally, the alkaline reduping should be pure.

    References: [1] Wang Peng, et al.
    A case of disseminated dermatitis blastomycosis[J].
    Chinese Journal of Internal Medicine, 2006.
    [2] Zhao Tianru, et al.
    Identification of positive bacteria with acid-fast staining and identification of false positive foreign bodies.
    Journal of Diagnostic Pathology (2003).
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