Tuberculosis diagnostic investigations and Tuberculin test

T.B is mainly a bacteriological diagnosis by Ziehl-Neelsen stained smear or culture on Lowenstien Jensen Media or Middle brook).
The culture on middle brook needs short duration (2-3 wks) PCR is also a recent methode .

1- Bacterial examination :
If -ve for 3times, this may indicates -ve T.B infection ?
• Mycobacteria is recognized by their surface lipids which makes them acid fast in the laboratory examination.
• Isolation of organism from

2-Radiological picture: (usually gives apical lesion)
* Cavity * Fibrosis * Consolidation
* Effusion * Collapse * Miliary shadow

3-Tuberculin skin test:

• It is used widely to screen certain high risk populations, particularly those who have been exposed to an infectious patients.
• The test involves an intradermal injection of the purified protein derivative (PPD) of the bacilli.
• After 48-72 hours the injection site is examined for visible and palpable induration.
• Because of a possible cross reaction after exposure to other mycobacteria a single tuberculin test to determine sensitization to mycobacterium tuberculosis is considered positive only if the diameter of the induration at the skin test site measures :
 * > 15 mm in immunocompetent individuals.
 * > 10 mm in sick persons without depression of their immune system .
 * > 5 mm in immunocompromised patients e.g (organ transplant recipients or patients with HIV infection).

Values of tuberculin test:

1. Positive test indicates recent or old infection or vaccination.
2. If it becomes +ve in a child, this mostly indicates recent tuberculous infection !?
3. For contact with a case :

4. A repeatedly negative test after 6 weeks from the onset of symptoms may rule out tuberculosis !?
5. Tuberculin test is also positive in atypical mycobacterium infection .
* Specific PPD for (avium,kansasi) is the method to differentiate atypical mycobacterial infection from mycobacterium tuberculosis.

Causes of false -ve tuberculin test:

1- Before 6 weeks (preimmune period)
2- Immunocompromised patient (anergy) e.g.  AIDS, Steroids, Cytotoxic, Sarcoidosis
3- Bad technique
4- Miliary T.B
5- Viral infection e.g measles ===> immunosuppression.

4- E.S.R :
- ESR is High in active T.B. (usually> 100)
- It is used in follow up
- Can rule out active T.B if it is normal !?

5- Blood picture
• Leucopenia with relative lymphocytes
• Anemia of chronic disease (normocytic, normochromic)

6- PCR : Recently it is an accurate technique(sputum - BM - CSF - urine).
7- Biopsy from the pleural, lymph nodes or solid lesion within the lung or from peritoneum, liver or bone marrow in disseminated disease .
Important links to related articles
-Pulmonary Tuberculosis def., causes, risks, pathology
Manifestations of Pulmonary Tuberculosis (symptoms, signs)
- Miliary tuberculosis manifestations and investigations
- Complications of pulmonary tuberculosis
Tuberculosis diagnostic investigations and Tuberculin test
Dr.Tamer Mobarak


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