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    Can a skin test detect tuberculosis?

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Posted on Wed, 1 Jun 2016 in Skin Hair and Nails
Question: I have some inquiries about TB skin test. I work with children and its required to do this test. I have done a skin test 3 years ago and it was positive then did blood and chest test as well. the doctor said everything look fine. so now I'm did updated test and it was negative this time, just with some skin irritations. is this normal? and what are the possible causes of negative result?

thank you
doctor
Answered by Dr. Shuba Hariprasad 4 hours later
Brief Answer:
Tuberculin test is a screening and nonspecific test

Detailed Answer:


A PPD (protein purified derivative) or the Tuberculin test for tuberculosis (TB) is done to see if an immune response to the bacteria that cause TB has developed.
It is used as a screening test, not a diagnostic test and does not necessarily mean you have TB.
People who have been vaccinated with the BCG vaccine can also have a positive result as those with a latent TB infection. It can also be positive if there is an infection by a bacteria that has a similar structure to the Mycobacterium that causes TB.

A negative result usually means that there is no immune response to the bacteria and that there is no active/latent TB infection. It can also be negative if there is any immunocompromise.

A negative after a positive could mean
: either a false positive or false negative (one of the results you encountered was false)
:or the initial immune response has diminished (happens after a vaccine in childhood with a temporary flare up in response to a similar infection, or infection by a similar bacteria that has recovered).

The above is based on whether the test was performed (administered and interpreted in the correct way and within the right time frame) and that you are a healthy individual without any immunocompromise.

I see some redness in the photo you sent. Can't make out any induration (not clear).
As mentioned above, this is a screening test and needs to be correlated with clinical findings and blood investigations to arrive at a diagnosis

Hope this information helps.
Please let me know if you have any further questions that I can help you with.
If not, please close this discussion & rate my answer.
Wish you good health.

Regards,
Dr. Shuba Hariprasad
Above answer was peer-reviewed by : Dr. Prasad
doctor
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Follow up: Dr. Shuba Hariprasad 24 minutes later
I was reading up on some info on result of tb skin test. I'm very worried. I was tested for hiv after 5 weeks from the sexual encounter, and it did come as negative. but then I heard that it is not 100% accurate. that I have to wait 12 weeks to have accurate results.

now I read online that negative tb skin test result could mean the person has hiv, so now I'm worried because I have still have that rest of per cent of inaccuracy because I only waited 5 weeks to do the test.
doctor
Answered by Dr. Shuba Hariprasad 1 hour later
Brief Answer:
Negative skin does does not indicate HIV infection

Detailed Answer:
Hello,
Thank you for the follow up.
Sorry for the lack of introduction earlier.
I'm Dr. Shuba Hariprasad and will help you with your query today.

HIV infection need not necessarily result in AIDS(adult immunodeficiency syndrome). A negative after a positive test may occur in immunocompromised people (AIDS is one situation, can occur due other disease as well) or could just be a false positive.
I understand that your concern is of contracting HIV. Please recheck with a Elisa test for HIV to Confirm. Do not panic based on a negative tuberculin test. 5weeks is not enough time for you to develop an immunodeficiency status and it is unlikely to be the cause of your negative Ppd test.

Hope I've reassured you.
Please let me know if you have any further queries.
If not, please close this discussion & rate my answer.
Wish you good health
Regards,
Dr. Shuba Hariprasad
Above answer was peer-reviewed by : Dr. Prasad
doctor
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Follow up: Dr. Shuba Hariprasad 35 hours later
for Elisa test how long do I have to wait to get a 100% result back?
doctor
Answered by Dr. Shuba Hariprasad 40 minutes later
Brief Answer:
3 months (99%), Maximum 6 months. More sensitive tests available (99%@2wk)

Detailed Answer:
Hello,
Thank you for the follow up.

A window period is the time between contracting an infection to the time that the infection shows up as positive in a lab test
In case of HIV this is usually 2-8weeks (95% test positive in 6weeks; 99%by 3 months) but in certain cases it can be upto 6 months.
To answer your question, an ELISA test for HIV can be reliable and taken as negative if the test is negative after 6 months from exposure.
A fourth generation HIV EIA has a better capacity than a Third generation one in detecting an HIV infection earlier

I know that this is frustrating and you would like to know sooner

There are other tests that can tell you sooner.
- a viral RNA test detected by pooled nucleic acid amplification test (NAAT) which can detect an infection as early as 2 weeks and 99%by 6 weeks

-p 24 antigen (detected by the fourth generation ELISA) by 3-5th week.

If you have a high risk of developing an HIV infection (partner known to have HIV or high risk for HIV) please inform your primary care physician who could assess you for signs of seroconversion such as
-aching muscles/joints
-swollen lymph glands
-sore throat
-high temperature
-rash
-fatigue
And can ask for a more sensitive test as mentioned above to detect an infection if present


Please note that these are nonspecific symptoms and have to be considered as part of the bigger picture, not as diagnostic criteria for HIV infection.

Since it has been already 5 weeks, I would advice to do a repeat ELISA (fourth generation) at 12 weeks to confirm. If you have any of the symptoms mentioned above without an obvious reason (like sore throat, fever in pharyngitis , for instance), please see your physician for a requisition to have a more sensitive test.
Please discuss the need for more sensitive tests with your doctor as they can help you decide if it is necessary based on a clinical examination and sexual encounter history.

I hope this helps.
Please let me know if I can help you with anything else.
Wish you good health.

Regards,
Dr. Shuba Hariprasad
Above answer was peer-reviewed by : Dr. Arnab Banerjee
doctor
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Follow up: Dr. Shuba Hariprasad 2 hours later
Ok thank you Dr. XXXXXXX
doctor
Answered by Dr. Shuba Hariprasad 4 hours later
Brief Answer:
You are welcome

Detailed Answer:
Hope I've been helpful

Above answer was peer-reviewed by : Dr. Raju A.T
doctor
Answered by
Dr.
Dr. Shuba Hariprasad

General & Family Physician

Practicing since :2002

Answered : 1087 Questions

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