The tuberculosis skin test (TST) is a simple and valuable tool for detecting tuberculosis exposure and infection. But testing positive does not always indicate TB.
TB (an abbreviation for Tubercle bacillus) is caused by Mycobacterium tuberculosis; a slow growing, wax-coated acid-fast bacterium that can remain viable in dried aerosol droplets for eight months. Although not considered highly infectious, it takes few M. tuberculosis bacteria to start a disease process that can destroy lung tissue, disseminate and result in death.
The Mantoux Tuburculin Skin Test
Those in the medical field, and others with a high risk of contracting tuberculosis, are routinely tested with the Mantoux tuberculin skin test (TST). The TST involves injection of a minute amount of tuberculin purified protein derivative (PPD) into the inner surface of the forearm.
How the TST Works
The immune system of those infected with the Mycobacterium tuburculosis will react to the PPD protein, which is an antigen derived from the bacteria's cell wall. For those with TB, the TST mimics reexposure to the pathogen, and reveals the rapid immune response of antibodies that would be present in a person who had previously been exposed to TB.
The test is read 48 to 72 hours after administered. Interpretation should only be done by a trained medical professional and involves evaluation of the injection site for a hard, red swelling indicative of a positive reaction.
False Positive TB Test
A positive TB test does not necessarily mean that a person has tuberculoses. There are several factors that can produce a false-positive TB skin test (TST) reaction.
Infection with Nontuberculous Mycobacteria
Although there are other, less common species within the same genus that can also cause TB, there are many other species of Mycobacterium that do not cause tuberculosis at all. A person who has been infected with another member of this genus may falsely test positive, since the PDD antigen used in the TB injection is common to other nontuberculous Mycobacteria.
BCG Vaccination
Although not yet used in the US due to questionable effectiveness, BCG (Bacille Calmette Guerin) vaccination is a TB immunization. Past inoculation with this vaccine can result in a positive TB test.
Booster Phenomenon
The TB "booster phenomenon" occurs because the immune system of someone previously infected with TB may lose its ability to react to tuberculin over time. Years later, when that person is skin tested again, the new test will come up negative, but the act of testing reminds the immune system about the previous infection.
If TST tested again, within a year, that person may have a positive reaction, because the first skin test essentially jogged the memory or the immune system, in effect, boosting its ability to react to tuburculin. This phenomenon is mainly seen in previously TB infected older adults.
Bottom line, a positive tuberculin skin test is not sufficient to distinguish among those:
with a previous or current mycobacterial infection other than M. tuberculosis
who have been immunized with BCG
previously infected but with no current infection
with a latent TB infection
with an active TB infection
Furthermore, it is possible for an incorrectly administered test appear positive or for a properly administered test to be falsely interpreted as positive.
The Next Step after a Positive TST
Due to the real possibility of a false-positive Mantoux tuberculin skin test, a positive result may be followed up with a chest x-ray or CT scan to reveal if there is an active infection present through evidence of lung cavities, nodules and fibrotic scars. A sputum test can also be done to determine if there is active infection, indicated by the presence of pathogenic Mycobacterium growing in strands, called cords.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
The copyright of the article What Causes a False Positive TB Skin Test? in Human Infections is owned by Tami Port. Permission to republish What Causes a False Positive TB Skin Test? in print or online must be granted by the author in writing.
i had to get one on friday....so far its negitive...i have to go back on
monday to the doctor for a check up
Jan 16, 2009 6:29 PM
Guest
:
I had a test done the other day and it came back positive I then had a
chest x-ray taken but still awaiting the results I hope I don't have TB.
Jan 18, 2009 10:15 PM
Guest
:
My son had a tb test done 10 day ago and now his arm has a bump, Is that
normal and should I be worried? Everything I have read says the reaction
would be in within 48-72 hours, just a side note, he has had the stomach
flu all week any relation?
Jan 24, 2009 10:49 PM
Guest
:
my gf had a test done a few years ago and it came back negative. then she
had it done a few days ago and it came back positive, had xrays and doc
said it wasnt contagious, but needs to be on medicine for 9 months. any chance to spot a false positive?
Jan 29, 2009 2:09 AM
Guest
:
I had to get a skin test on monday, had the results read today and it
showed I was positive for it. This has never happened to me! I am having my
chest X-ray tomorrow, God knows this is one more thing I don't want to
worry about! I wasn't scared, but reading everything online has honestly
freeked me out...I hope I don't have it!
Jan 31, 2009 2:53 PM
Guest
:
I just got a TB skin test and it came back positive but i have none of the
symptoms so is the test false
Feb 3, 2009 1:14 PM
Guest
:
I had a tb test done 2 weeks ago and it was negative. I forgot to have it
checked so I went yesterday to have it readministered and the girl injected
the ppd in the exact same spot as the first test and now I am having a
positive reaction. Could the second injection be reacting to the first
causing a false positive?
Feb 4, 2009 6:24 AM
Guest
:
Since I had 2 or 3 BCG vaccinations while growing in the republic of
Georgia and than living in Russia, my TB skin test is always shoving
positive. Recently, I had one TB skin test again and no surprise it was
positive. I had an X-Ray a week ago that shoved no evidence of
tuberculosis. Yesterday I've received a letter that I need to take a
medication for 9 months. I heard that medication makes you feel sick. Any
advices on taking or not this medication? I do not understand why I need to
take medication if I had several inoculations?
Feb 14, 2009 6:53 AM
Guest
:
Guys, don't get panic if you ever get a positive PPD test. thousands of
tons of people in US and other countries give a posite ppd test. I just got
mine and turn out to be positive. I'm taking my x-ray on monday, most
likely it will be negative, but still will be taking pills for 6
months(Izoniazid 300mg/d) and piridoxine(vit B6) 10mg/d to avoid possible
nerve problems. only about 10% of people with latent(dormant)
infection(ppd+ patients) will develope active TB disease, those people will
be most likely patients who are immunocompromised(bad immune system) such
as HIV positive patients, patients going throught chemotherapy, or elderly
patients with different medical complications. so if you are positive, just
take your pills for a couple of months and then enjoy life the way you
want. who's talking? sorry if I did not introduced myself I'm a PGY 2
internal medicine resident. good luck people.
Feb 19, 2009 11:04 PM
Guest
:
I ha had my TB test done 2 days ago and I went in today to get it read and
I tested positive. I really don't know anything about this, but i were to
have would i still be able to work?
Feb 21, 2009 7:43 PM
Guest
:
i am a teacher and tested positive. after having a chest xray which was
negative I was able to go back to work with a note from the doctor stating
I had latent TB. now i guess i should take the meds for 9 months...
Mar 16, 2009 3:35 PM
Guest
:
I am an RN who does these every day. If you were exposed a long time ago
or had a BCG vaccine a long time ago, your antibodies can go down over
time. Each TB test "boosts" your antibodies. When you are
exposed to someone with active TB, they cough in your face, you breathe it
in and it goes to your lungs. Your immune system essentially says
"that's not supposed to be there" and walls the infection off to
protect you. When it does this you form antibodies, which cause the test
to be positive. You are not contagious, no sick, and it doesnt show on
chest xray. Unfortunately at some point later on in life, when your immune
system is depleted etc... you can come down with active tb. You are most
likely to become active in the first 2 years after exposure, but can still
become active later. This is called latent tb. They treat this with
mainly INH for 6-9 months. Your physician should monitor you while taking
this med. This will prevent that later on in life activation. There is a
new blood test called a Quantiferon Gold that better distinguishes from a
false positive TB skin test. If positive, it is more likely from exposure
than BCG vaccine. If negative it is more likely BCG activation. They do a
CXR when you go from neg. to pos. to make sure you don't have active tb.
Stomach flu should not affect it. Immune system problems or a live vaccine
or an improperly administered test all can affect the results. Different
kinds of mycobacterium can also make the test positive. Most do not have
tb unless they have symptoms. Symptoms are cough lasting longer than 3
weeks, coughing up mucous/blood, unexplained night sweats (not menopause);
unexplained weight loss. Usually you are quite ill. Hope this helps.
Mar 24, 2009 12:24 PM
Guest
:
I just got tested for TB yesterday and my job. A few hours it went down and
it looked like the one i had gotten before which was negative. Then a few
hours later It formed a bump. Then this morning the bump went down but i
have a big red circle. I am going to get it read tomorrow. I hope I don't
have it....
Apr 3, 2009 3:44 PM
Guest
:
I am getting a Immigration form filled out and yesterday my test came back
positive it freaked me out because I have no idea what it is or even where
i could of gotton it!! i had to do an xray yesterday also but the didnt
give me any results so i was wondering if it was something serious why
didnt they prescribed some kind of antobiotic or something I dont knw whats
going on and I have to pick up my Immigration physical form frm them
monday!!! Since then i havnt bin able to sleep...I read up on the symptoms
but i dont have any of them this is giving a big headache??? I dont want it
to interfear with my form plus this is my health and im very worried!
May 2, 2009 1:51 PM
Guest
:
I just had a positive ppd test (18 mm) and a negative chest x-ray. So many
different opinions on what to do from here. Some Dr.s say take the 9 month
treatment, some say it isn't necessary. I wish there was a standard
procedure. I guess it's up to me to decide. Weigh the pros and cons from
taking or not taking the medication.
May 15, 2009 9:58 PM
Guest
:
i just did an X RAY and found something in my lung and suspecting it's TB.
I just did the PPD jet yesterday and IF, after 2 or 3 days it turns out
positive but i still DO NOT HAVE any symptoms of TB. Am i really infected
or will it probally still just scars or normal tissue in my lung? I could
not apply for my student visa due to this problem and i hope i do not need
to go for the 6 months medication. Do i really need to go for medication.
Hope it turns out negative for everything.
May 18, 2009 7:48 AM
Guest
:
my son (13 year old) had a test done eight months ago and it came back
negative. then he had it done three weeks ago and it came back positive,
had xrays and doc said it wasnt contagious (normal,clean), but needs to be
on medicine for 9 months.Since he had one BCG vaccinations after his born
in the republic of iraq . any chance to spot a false positive?
Jun 2, 2009 6:03 AM
Guest
:
I tested false positive about 13 years ago when I was 10. Since I had no
risk factors whatsoever (didn't know anyone who traveled, never traveled,
never even took public transportation) and my x-ray was clean, they
re-tested and it came out negative. I didn't have to take anything. My
sister is a doctor and she said now they would have made me take the meds
(they may also have been lenient due to my age). However, there are meds
to treat TB, and if you have a positive PPD and no active symptoms you
shouldn't have to worry about any long term health effects from TB. TB
kills from lack of treatment.
I'm taking one tomorrow. Work in
the health field, so I just don't want a positive to screw up my new job.
always paranoid since that false positive. :)
Jun 7, 2009 2:59 AM
Guest
:
My husband has had the HBC vaccine because he is British. He went for a new
job in the states and they made him take the skin prick test last
Wednesday. Of course, it has come back positive since he has the antibodies
build up in his body against it. Since last Wednesday, all he has done is
experience syptoms of TB such as a wretching cough and lock jaw. He went
for a chest X-ray on June 4th which wont be read till this coming Wednesday
the 10th. I am worried this is harming his lungs. Any suggestions on what
we should do? I will check back later.
Jun 13, 2009 4:08 PM
Guest
:
I had my TB skin test last thursday and they had it read today..I was
positive. I can't beleive it! Before migrating here in the States I had my
Chest X-ray and I was negative..Im a little depress because I need to have
a " negative" tb result for my prospective job.I have to attend
an orientation next week.Might as well have a Chest X-ray and I hope it
will be negative..
Jun 17, 2009 10:44 AM
Guest
:
Ok I took a PPD Skin Test last August before going of to school and it came
out negative. However I just recently came back from school and took the
test again for a job and it came out postive. Immeadiatley after I took a
Chest X-Ray and my heart and lungs were perfectly clear. The doctors want
me to be on medication for 9-12 months but my parents and I think its best
not to take the medication due to the side effects. I really think that the
nurse read my test to early and my aunt who is a nurse at a hospital in
boston feels the same way. Should she should have read it in 72 hours or 48
? I am stressing over this because I really dont want to be take this
medicine and if i dont what are the consequences ? Help me with what to do
please ? !!!!!!!!
Jul 2, 2009 8:43 PM
Guest
:
I HAVE A FRIEND WHO IS 21 YEARS OLD AND SHE TESTED POSITIVE ON THE TB TEST
AND SHE ALSO HAD AND XRAY DONE. SHE IS 4 MONTHS PREGNANT HOW DOES THIS
AFEECT HER AND HER UNBORN BABY,SHE WAS TOLD SHE NEEDED TO TAKE MEDICATION
BUT HAD TO WAIT TILL THE BABY WAS BORN AND HOW CONTAGIOUS IS THIS? DO YOU
HAVE TO PREVENT YOURSELF FROM KISSING YOUR PARTNER OR DRINKING FROM THE
SAME CUPS OR DOES THIS HAVE NOTHING TO DO WITH TB POSITIVE?
Jul 3, 2009 3:35 AM
Guest
:
Hi, YAh I understand everyone here and how scared we all are. I just tested
positive for "latent TB", they found it in an X-ray, not much was
there, just a little bit these small dime sized white things, they called
my scars "hard scars" meaning that its better then soft scars.
I'll hand in my China Z Working Visa Health report to the Chinese
immigration next Month. I guess from what I read here, all hope is lost, I
guess I'll never be able to become a teacher again in Asia since I can't
pass the health tests any more due to "latent TB", My life just
ended.
I wish everyone good luck, i hope we all can be cured
some day, God speed.
To all English Teachers, I guess its best
to start wearing masks now, I'm a health young 20 year old, now I might not
be able to work as an English teacher ever again, wear the mask.
Aug 2, 2008
