The Tuberculosis Test – How to Identify the Disease

The tuberculosis or TB infection is an airborne disease caused by the bacterium Mycobacterium tuberculosis. It can turn fatal if left undiagnosed and untreated.

The best way to get a solid diagnosis for this condition is by undergoing a tuberculosis test.

tuberculosis test

Signs like night sweats, persistent coughing, and malaise indicate the need for a tuberculosis test.

You should see a doctor for a possible diagnosis of tuberculosis (TB) if you have the following symptoms:

  • A persistent cough for three weeks or more, with or without blood in the sputum
  • Fever
  • Profuse sweating at night
  • Malaise and weakness
  • Loss of appetite
  • Weight loss for no apparent reason

The complete medical assessment of TB will include a physical examination, a chest X-ray, a microbiological examination of your sputum, and an evaluation of your medical history. But the most important diagnostic tuberculosis test would be the Tuberculin Skin Test.

The Tuberculosis Test – Assessment Via Physical Examination

In case you have a suspected case of TB, conducting a physical examination helps your doctor look for the signs of this infection. He uses a stethoscope to listen to your breathing for unusual sounds that may indicate some form irregularity in your lungs. In addition to checking you for symptoms of active TB, the doctor will observe you for signs of a TB infection (such as swollen lymph nodes) in parts of the body other than your lungs. This is done to identify the possibility of extrapulmonary TB.

Evaluation Of Medical History

When you visit a doctor for a possible diagnosis of TB, one of the first things he is likely to ask is your medical history. His aim is to collect information to decide if you are susceptible to TB. Here are some of the questions you might be asked as part of this consultation.

  • Have you been in any situations that might have increased your risk of being infected with TB-causing bacteria? For example, have you had contact with a person who has active TB?
  • Have you recently travelled to places where TB is prevalent?
  • Have you had an HIV test in the past 6 months?
  • Are you currently taking any medicines, both prescription and non-prescription, including herbs and natural products?

The physical exam and the evaluation of your medical history and are helpful in making a preliminary suggestion if you have active TB. But the conclusive proof is the presence of the TB-causing bacteria in the sputum from your lungs.

Microbiological Examination Of Sputum

The sputum test helps detect the presence of the TB-causing bacteria that are infecting your lungs or your respiratory tract. A smear microscopy of sputum is often the first TB test that is used in countries with a high rate of TB infection.

As part of this test, a sample of sputum is collected from the person showing TB signs and symptoms. A very thin layer of the sputum sample is placed on a glass slide and spread thinly. This layer is called a smear. Stains (for example, acid that’s fast staining using Ziehl-Neelsen stain) are then applied to the sample, and the slide is examined under a microscope for the TB bacteria.

Tuberculosis Test Using The Tuberculin Skin Test

This is the standard method used to screen people for a TB infection. It is also called the Mantoux tuberculin skin test. It is performed by injecting a small amount of a purified protein fluid called tuberculin into the inner surface of the forearm.

Within 48 to 72 hours a reaction might occur on the arm. After this time period, the doctor looks for a raised, hard area or swelling on the forearm. If such a sign is visible, the doctor measures its size in millimeters using a ruler.

The results of the tuberculin test are classified as follows:

An area of swelling of 15 or more millimeters is considered as a positive indication of TB in any individual, including those carrying no known risk factors of TB.

An area of swelling of 10 millimeters or more is considered a positive sign of TB in

  • Recent immigrants (< 5 years) who have come from countries with a high prevalence of TB
  • Drug users who employ syringes for injecting drugs
  • Those who live or work in high-risk conditions
  • People who work in mycobacteriology laboratories
  • Children, including infants and adolescents, who have been exposed to adults in high-risk categories

An induration or thickened tissue of 5 or more millimeters is considered a positive TB case in

  • HIV-infected persons
  • Individuals who’ve had recent contact with a TB-infected person
  • Patients with organ transplants
tuberculosis skin test

If the tuberculosis skin test results in the hardening or swelling of the forearm tissue, it usually indicates tb.

Chest X-Ray

Acute pulmonary TB can be easily seen on a chest X-ray, through patchy infiltrates of the infection. As a result, a chest X-ray is used for diagnosing TB.

The tuberculosis test is more of a combination of these assessments than a standalone test. It helps confirm the presence of the TB infection, while avoiding the chances of certifying false positives as a case of tuberculosis.


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