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 tuberculosis. 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.
Jul 21, 2009 9:23 PM
Guest :
went for a tb skin test on saturday,went for a check up yesterday, turned
out to be positive, hat to go get a chest x-ray done the same day, and got
results in today! x-rays indicated I am negative. doc said i was exposed to
someone who has tb thats test read to be positive.
Jul 28, 2009 7:13 PM
Guest :
I got my first shot last week, it was negative. Got second today, and it is
red, swollen, hard and itchy...
Jul 31, 2009 7:35 PM
Guest :
Me and my fiancee were just tested today the bump went down but our friend
has tested positive and he stayed over our house recently his is not in his
lungs though, it started on his skin we have a wedding next month we go for
the results on sunday i hope we dont have it!!!
Aug 10, 2009 10:47 AM
Guest :
My dad younger sister was in the hospital [intensive care] for 1 month with
TB, she was such a fighter as she survived 2 heart attacks and several
other stuff.
Unfortunitly she died today aged 49. Reason
why i am writing is to see if anyone could help me... when i was
visiting herin the hospital i always kissed her when i was leaving, last
time i saw her was Thursday. On friday nighti started having this bad
cough, pnumonia, sweats at night, dizzy spells! Could anyone tell me if i
could maybe have caught anything from her as i read further up the page
that if you were with someone close enough you can catch it that way? If
anyone could help me either email me or reply on here, As i would be realy
greatful. thank-you ps: c_26.03.88_d@live.co.uk !
Aug 14, 2009 10:01 AM
Guest :
i had a BCG vaccine years ago when i was an infant, i know that if I've had
the shot my TB skin test would be positive and that's why i would have to
do the chest x-ray instead. However, last year i didn't remember if i had
my BCG shot so i just went with the skin test... i came out negative. I had
a skin test again this year and am currently waiting to have it read. I
just want to know... why was i negative despite my previous vaccination?
Aug 17, 2009 1:44 PM
Guest :
Whenever I had a TB test done, the next day it was already not visible
anymore. Last month after my test, I developed a red itchy circle and a
slight bump.... though my doctor and a nurse decided the test was negative
........ I didn't believe that (mostly because I had a cough for 6 weeks
now and neither bacterial antibiotics, nor fungal meds helped) and so I
demanded 3 days ago another TB test - which came out positive today....
went already for chest x-ray but still waiting for the result.
Aug 20, 2009 6:47 PM
Guest :
I tested positive about 4 years ago on the skin test (was working in a
hospital). Chest x-ray came back fine. Started on the INH meds and it's
no big deal! No side effects whatsoever. They did tell me though not to
bother taking another skin test and going through it all again, they gave
me a laminated card that was proof I'd completed the course of INH meds.
If you have questions/concerns people, talk to your healthcare
professional. That's what they're there for. I plan on going into
Nursing. This will not hold me back. Be informed, not scared!
Aug 21, 2009 7:05 PM
Guest :
i've been reading your comments i have 3yr old that tested positive,chest
xrays tomorrow... what are the side affects of the pills that your talking
about?
Aug 26, 2009 7:52 PM
Guest :
Last month of July I took a first TB test and the result was Negative, then
I took another test again after that my right arm is bump but i'm not sure
what's the result so I have to go there and have to read by the nurse. I'm
so excited about it but im a little bit nervous. hopefully it's still
negative just like the first one
Sep 3, 2009 11:07 PM
Guest :
I just recently had a TB test. When the nurse injected the PPD just below
the skin, she was not able to get the desired little round bump of PPD.
She just said "oh well" and didn't seem too concerned. When I
returned 2 days later there was a redness (not raised)and the spot was
measured. It was about 10mm in length. I went and had an x-ray, and my
x-ray was clean. Could this be a false positive reading? My sister is
very strongly insisting that I should go and get another test administered.
What do you think?
Sep 6, 2009 4:40 PM
Guest :
I had a test on thursday, and today it is red and Swollen ...should i be
worried.??
Sep 18, 2009 9:20 AM
Guest :
i had a positive test too and had to get the chest x-ray done. However,
since it is "the first positive test" (in my record) i had to
choose between taking two meds, isoniazid (INH)for 6 months or rif
(rifampin) for 4 months. I chose rif. I just completed 1 month, i feel a
little-off, but ok so far....the doctor stated that i might have had
positive test because of the BCG vacination that i might have gotten when i
was young. BCG was (is ?) common in Europe, asia and african countries.
Best of luck folks.
Sep 23, 2009 12:13 PM
Guest :
I am 6 months pregnat, and I had a skin TB test conducted and it came back
positive, then I had to get a chest x-ray exam, and that came back
negative, my doctor says that i might just had been exposed to tb but i am
not carrying the active viruse. I mad the mistake of telling my inlaws and
they insist that i get treated for it. after they found out my skin test
was positive they all went to get tested. even after i told them that the
x-ray showed a negative result. they are acting like if carrying the black
plegue. i dont know what else to tell them to get them of my back.
Sep 25, 2009 6:25 AM
Guest :
Read all these comments.Is there no one to answer them? If America
doesnt appreciate BCG vaccination why making people take them before
granting them their visa? It's so confusing to Asians,Europeans and
even the Africans in the states.
Oct 12, 2009 11:13 AM
Guest :
i got tested for TB and went back the NURSE said it was a negative but i
see a bump and its red. SHOULD I BELIEVE THEM?
Oct 15, 2009 5:46 PM
Guest :
i had my tb skin test result today and it came out positive. i was really
shock! after reading all the comments here i feel much better, at least i
am not alone. i don't know if i should tell my family and relatives because
i don't want them to think that i am contagious or something. i will have
my x-ray to confirm the result. i hope it will be fine because i had this
test due to a new job. i don't want to take those medication for 6 months.
i really feel bad today.
Oct 26, 2009 11:37 AM
Guest :
i had a test on a tuesday read on a friday. they sent me to a doctor for
xrays. he didnt think the first test was positive. so he gave me another
test had reaction of 2cm. but because of the first test he had me take
chest xrays which were neg. because he is confused by this i have to go to
tb clinic to see what they say. i have no symtoms of this dease.
Oct 29, 2009 8:31 AM
Guest :
I got tested for TB and it came back positive. They ordered a chest x ray,
but I won't know the results until tomorrow. I am scared and freaking out.
I needed the test for my new job. I am from a foreign country and had the
BCG vaccine and boosters while living in my native country. Could the
positive result be due to this vaccination? HELP PLEASE....!
Nov 5, 2009 1:21 AM
Guest :
Hi everyone,
For more info, go to the CDC or other government
websites. If you had the BCG vaccine, you can have a false positive because
you now have titers. Sometimes, if your body hasn't been exposed to TB,
your immune system can be "dormant" and your PPD can be negative
at first. If you get another one a year later, your immune response
"remembers" that you have immunity and can react a second time as
positive. Also, if you want to get retested, you should wait at least 4
weeks or that can give a false positive. If the doctor recommends taking
the medication you should. Latent TB means your not contagious, but you
carry it. Later in life you can activate and have full blown contagious TB.
Taking the meds clears you of any TB and you will no longer have TB. Side
effects are not as bad as they seem and the risk of becoming contagious is
not worth it because you could go on to infect others. Otherwise, it is
like cat hing any other germs. Just make sure you follow up on your health
and do what is recommended by your healthcare provider. Also, some people
have a false positive due to an allergic reaction and not from an actual
reaction to the TB proteins.
Aug 2, 2008
