Author Topic: False positive on a TB test?  (Read 42548 times)

Offline Oil Lady

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False positive on a TB test?
« on: September 11, 2011, 10:06:04 AM »
Back in January I took (and passed) a TB test so I could enroll in a CNA course. It was the standard injection-in-the-arm thing (in my left arm), and then you report back after 48-72 hours for a nurse to check it.

Two weeks ago, a nursing home hired me so I had to take ANOTHER such test for pre-hire screening. They administered the TB test themselves from their own in-house supply of those injections. This time I got the injection in my right arm. Once again, I passed (in fact is was a week ago today --Sunday-- that I showed my arm to the nurse at the nursing home and she smiled and waved her hand and said "Oh you're fine!").

This past Friday I went through orientation, and was told I needed one more TB test as a new-hire (the one last week was just pre-hire, this new TB test this week is new-hire). So, Friday at 3:00 PM, Eastern Time they injected me again and this time in my left arm.

Well, today it is now ALMOST 48 hours since that injection, so I am entering the 48-72 hour window when I need to have the injection site inspected. And I am showing what I think is a borderline positive reading. It's the size of a quarter, and is mildly red, VERY hard, and mildly raised. It FEELS like the thick hardened part descends down into my skin about 3 millimeters. The hardened part that ascends above my skin is about 2 or 3 millimeters.

If this is a positive reading, I can only draw three possible conclusions:

1) Last week was a false negative.
2) This week was a false positive.
3) I caught TB somewhere between last week and this week.

I have to report to work today for a 3-11 shift, and that's also when the nurse will check my arm --and 3:00 this afternoon is going to be the start of the 48-hour mark from the Friday injection.  The nurse who will be on duty this afternoon to check my arm is the exact same nurse as last week. In fact, I worked last night with her, and she remembers me and remembers last week's negative.


Offline Cedar

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Re: False positive on a TB test?
« Reply #1 on: September 11, 2011, 10:15:27 AM »
Your body reacting from all the other pokes?

Cedar

Offline hanzel

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Re: False positive on a TB test?
« Reply #2 on: September 11, 2011, 10:37:39 AM »
not being a Dr, nurse, or any other medical person, I would ask.. arnt the last to test a little to close apart ?  if i remember correctly .. you were just given a dead form of TB to see how your body would react ( like the dead flu shots they have each year ) so yes .. you are currently fighting TB in your body. i know my wife has a problem with the TB vaccine she was given as a child ( its not longer available ) she will always show a false positive now on the TB test.  a few years ago I had to have a yellow fever vaccine to travel and for a week after, I "had" yellow fever.

Offline cheryl1

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Re: False positive on a TB test?
« Reply #3 on: September 11, 2011, 10:45:46 AM »
You are allergic to the test. It was negative the first time because you didn't have any anti-bodies to cause a reaction. You will test positive for TB the rest of your life, but you don't have TB. Wherever you applied will probably want a chest x-ray to confirm that you don't have any active disease before they hire you. They pay for that, not you-it's part of the pre-employment screening. 

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #4 on: September 11, 2011, 12:10:20 PM »
Well, I'm off to work. I'll see what happens.


Offline Pathfinder

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Re: False positive on a TB test?
« Reply #5 on: September 11, 2011, 02:08:20 PM »
I remember in school false positives to the test were common - not a majority but a sizeable percentage. I agree with the above - you got them too close together. Sounds like an example of people going through the motions of their job without actually thinking about their jobs!

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #6 on: September 11, 2011, 07:35:10 PM »
Back in January I took (and passed) a TB test so I could enroll in a CNA course. It was the standard injection-in-the-arm thing (in my left arm), and then you report back after 48-72 hours for a nurse to check it.

Two weeks ago, a nursing home hired me so I had to take ANOTHER such test for pre-hire screening. They administered the TB test themselves from their own in-house supply of those injections. This time I got the injection in my right arm. Once again, I passed (in fact is was a week ago today --Sunday-- that I showed my arm to the nurse at the nursing home and she smiled and waved her hand and said "Oh you're fine!").

This past Friday I went through orientation, and was told I needed one more TB test as a new-hire (the one last week was just pre-hire, this new TB test this week is new-hire). So, Friday at 3:00 PM, Eastern Time they injected me again and this time in my left arm.

Well, today it is now ALMOST 48 hours since that injection, so I am entering the 48-72 hour window when I need to have the injection site inspected. And I am showing what I think is a borderline positive reading. It's the size of a quarter, and is mildly red, VERY hard, and mildly raised. It FEELS like the thick hardened part descends down into my skin about 3 millimeters. The hardened part that ascends above my skin is about 2 or 3 millimeters.

If this is a positive reading, I can only draw three possible conclusions:

1) Last week was a false negative.
2) This week was a false positive.
3) I caught TB somewhere between last week and this week.

I have to report to work today for a 3-11 shift, and that's also when the nurse will check my arm --and 3:00 this afternoon is going to be the start of the 48-hour mark from the Friday injection.  The nurse who will be on duty this afternoon to check my arm is the exact same nurse as last week. In fact, I worked last night with her, and she remembers me and remembers last week's negative.

Medical testing (like life) is never perfect, there will always be false positives and negatives.  This is a frustration to all involved, but it's just the way it is and practitioners have to do the best we can with what we have.

The recommendation for incoming healthcare workers is for a two-step PPD (TB) skin test.  This is to make sure that those workers who test positive in the future were infected during their time of employment and not actually positive at the start of employment.  Sometimes individuals with long term latent TB infections or prior BCG vaccination will test negative on the first test, but the PPD reminds the immune system what TB looks like, so that subsequent testing is positive.  This is why they do two tests a couple weeks apart, to increase the chances of catching new employees with true positive skin tests.

If the nurse who "read" your PPD test last week didn't actually feel your injection site she might have missed the induration which is the key to interpreting whether it's positive.  Redness is not meaningful, it's the hard, raised induration that is measured and determines whether a test is is positive. 

The tests have to be administered at the right depth, it must raise a wheal, or blister, and you can't press it with your finger or band-aid so that it squeezes the injection out before it can be absorbed.

It takes about 2-3 months from the time of TB infection for the body to produce a positive PPD skin response, so it is unlikely that you have been infected between the two most recent tests.

There is a blood test for TB, but it doesn't have much better sensitivity or specificity than the skin test we've been using for years, so we don't have good guidelines on how it should be used.

Fortunately, most people who are infected with TB never have any active disease, because the immune system walls off the bacteria and keeps it under control.  About 5-10% of these people with latent infection will go on to active disease at some point in the future, usually in situations where there is some compromise of the immune system that allows it to multiply.  These individuals with an active lung infection are the ones that are a risk to the public's health, because their coughing transmits the particles to others in airborne droplets.

For healthcare workers who test positive on the skin test, the usual practice is to obtain a chest xray to ensure there is no active disease in the lung.  If the radiologist finds no evidence of disease, the worker is allowed to continue working and they will get an annual chest xray, instead of the skin test, for monitoring.  Individuals who have recently tested positive should be evaluated for possible treatment of latent TB, as this has been shown to greatly reduce the incidence of progression to active disease in the future.  There are risks involved with treatment, however, and the treatment period lasts for at least 9 months, so recommendations need to be made on a case by case basis.

Here's a CDC link for more info:  http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm

Offline The Professor

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Re: False positive on a TB test?
« Reply #7 on: September 11, 2011, 08:22:00 PM »
It happens. I test positive for TB rather frequently and just automatically ask for the x-ray, which shows that I'm clear.

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Offline Cooter Brown

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Re: False positive on a TB test?
« Reply #8 on: September 11, 2011, 08:49:36 PM »
If all else fails, go the the designated TB center for your area where they actually know what they're doing.

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #9 on: September 12, 2011, 07:52:37 AM »
Well guys, I went to work last night, the nurse on duty examined my arm, and she said "Oh, you're fine!" She also said a TRUE positive would be a monster-sized bulge on my arm, as if I was hatching an alien in there. So it should be good! 8)

Thanks for all your help, everyone!




Offline FreeLancer

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Re: False positive on a TB test?
« Reply #10 on: September 12, 2011, 03:26:01 PM »
It's the size of a quarter, and is mildly red, VERY hard, and mildly raised. It FEELS like the thick hardened part descends down into my skin about 3 millimeters. The hardened part that ascends above my skin is about 2 or 3 millimeters.

That's odd, because the reaction you so thoroughly described is induration of 24mm diameter, which is positive by any standard.

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #11 on: September 12, 2011, 03:52:28 PM »
Well, the nurse has the final say. :)


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Re: False positive on a TB test?
« Reply #12 on: September 12, 2011, 04:09:32 PM »
My wife has had false positives for several years and the longer you're in nursing the more likely you are to develop false positives.  Frequent testing tends to "cause" the false positives.

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #13 on: September 12, 2011, 04:52:49 PM »
Well, the nurse has the final say. :)

Yep!  It appears you may have slipped through the cracks this year. :-\

My wife has had false positives for several years and the longer you're in nursing the more likely you are to develop false positives.  Frequent testing tends to "cause" the false positives.

Anytime the number of screening tests is increased, the likelihood of false positive results is increases as well, but PPD does not produce a lasting immune response by itself.  It can awaken a dormant immune response to prior infection or BCG vaccination, which is the reason for performing two-step testing.

The problem with a positive test is that we don't know if it's true or false, which is why serial screening with xray or other lab tests is required.  It's rarely a good idea to assume that a positive test represents a false positive, without doing additional testing.  The vast majority of people with positive TB skin tests either have had prior BCG vaccination, or have a latent TB infection that most likely will never progress to active disease.  But since we don't know latent from active or BCG we have to proceed to xray to rule out active pulmonary disease, which is the mode by which TB is spread. 

From the employers perspective, they don't care (well, maybe a little) whether you have latent TB, have had BCG vaccination, or have a false positive result.  Their duty is to try to ensure that their employees don't have active pulmonary disease that can spread TB via airborne droplets.  So anyone with a positive skin test needs an xray.  If the xray shows evidence of active pulmonary disease, they need three negative sputum cultures before they can return to work.

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #14 on: September 12, 2011, 05:26:12 PM »
She's an awesome nurse, and we discussed her training, and then we discussed a web resource. According to the web resource, I needed to have a lump that exceeded 10 millimeters in height. So my lump -- while hard, red, and the size of a quarter-- only elevated above my skin by 2 or 3 millimeters.

Here's the web resource which indicates 5 mm, then 10 mm, then 15 mm of induration for different cases. And my case as a health care worker puts me into the 10 mm category. .

http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm





Offline FreeLancer

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Re: False positive on a TB test?
« Reply #15 on: September 12, 2011, 06:30:46 PM »
She's an awesome nurse, and we discussed her training, and then we discussed a web resource. According to the web resource, I needed to have a lump that exceeded 10 millimeters in height. So my lump -- while hard, red, and the size of a quarter-- only elevated above my skin by 2 or 3 millimeters.

Here's the web resource which indicates 5 mm, then 10 mm, then 15 mm of induration for different cases. And my case as a health care worker puts me into the 10 mm category. .

http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm

I'm sure the nurse is great, but I'm concerned she may not understand how to read a TB skin test properly.

Any hard reaction that is raised above the surface of the skin at the injection site is positive induration.  The height above the skin is not what is measured, it is the diameter of the indurated area across the longest axis.  Your post stated that the induration was 2-3mm above the skin and as big around as a quarter.  A quarter is definitely more than 15mm in diameter, which is why I'm surprised it was interpreted as being a negative test.  We don't measure height, we measure width.  We don't measure redness, we measure the indurated area.

At my clinic, too many positive TB tests were being missed and healthcare employers were not happy about it.  So now us docs are doing all the readings, at least a dozen per week. 

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Re: False positive on a TB test?
« Reply #16 on: September 12, 2011, 10:19:46 PM »
The problem with a positive test is that we don't know if it's true or false, which is why serial screening with xray or other lab tests is required.  It's rarely a good idea to assume that a positive test represents a false positive, without doing additional testing.  The vast majority of people with positive TB skin tests either have had prior BCG vaccination, or have a latent TB infection that most likely will never progress to active disease.  But since we don't know latent from active or BCG we have to proceed to xray to rule out active pulmonary disease, which is the mode by which TB is spread. 
After last year's PE, she's had just about every pulmonary test known to mankind, not to mention other negative TB tests (including her most recent two), so she was clear at her last hiring, but in her line of work obviously her opportunity for exposure is considerably higher than the normal population in the US.  I can't even imagine the infection rate for healthcare workers in third world countries. :o

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #17 on: September 13, 2011, 12:17:29 AM »
..so she was clear at her last hiring...

That's great!

I can't even imagine the infection rate for healthcare workers in third world countries. :o

As things continue to go down the toilet, and poverty and crowding become more widespread, our rates of infection won't be far behind.  We take for granted many of the public health measures we've been privileged with over the last century. 

Didn't the father in The Road die of TB?

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Re: False positive on a TB test?
« Reply #18 on: September 13, 2011, 09:09:59 AM »
As things continue to go down the toilet, and poverty and crowding become more widespread, our rates of infection won't be far behind.  We take for granted many of the public health measures we've been privileged with over the last century. 

Didn't the father in The Road die of TB?
Agreed.  Diseases of all sorts are going to come back in all sorts of ways because of malnutrition, lack of clean water, crowding, hygiene (and education related to hygiene), and worst of all, the loss of effectiveness of antibiotics.  Not a pretty way to watch things go, but nature isn't always pretty.

There's a lot of unanswers in The Road for me.  Could have been TB, could have been silicosis...

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #19 on: September 13, 2011, 03:19:45 PM »
Could have been TB, could have been silicosis...

Good call, silicosis is a major risk factor for progression to active TB.

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #20 on: September 13, 2011, 08:14:21 PM »
Dangerous TB spreading at alarming rate in Europe

TB is back with a vengeance.

http://www.reuters.com/article/2011/09/13/tuberculosis-europe-idUSL5E7KC2BR20110913

Offline leprachuan45

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Re: False positive on a TB test?
« Reply #21 on: September 28, 2011, 08:10:08 PM »
I actually had one that bled worse than normal and caused a small hematoma at the injection site. You maybe had the same thing.

Offline Ken325

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Re: False positive on a TB test?
« Reply #22 on: September 28, 2011, 09:11:03 PM »
I also test positive and have to request the x-ray.  My whole arm goes red and swells up.  It is kind of painful as well.  I  take a medication that has to be injected for another problem and I have noticed small swollen and red areas when I don't do a good job of cleaning the site of the injection.  My guess is that is what this was.

Offline kymedic

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Re: False positive on a TB test?
« Reply #23 on: October 02, 2011, 12:48:34 AM »
 +1 re: FreeLanceR's comments about the reads...it does sound positive and that she's not reading it right...it's amazing how something so simple can be scewed up so much, whether its injecting properly to get the wheal or doing the read. It's almost certainly a false positive but you need the CXR to be sure.

Also, you said that you got one of your injections two weeks ago and had it read one week ago...remember they must be read in the 48-72 hour window.

Offline Rorschach

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Re: False positive on a TB test?
« Reply #24 on: October 02, 2011, 07:11:11 PM »
The test you performed came in a box that might have a form with it that would describe the tests accuracy, and how to read the test.  Or if it was not in the box the company will probably have a form online since the FDA regulates medical tests as well as medications.   You should consider tracking this down since it may help you settle in your own mind if the test was performed and read accurately.  Regardless of whether the test is truly positive or negative you are in the health field now and need to do what you can to protect yourself.  In addition to good hand hygiene, and other precautionary measures, making sure your immune system is able to perform at its peak against this pathogen  will be helpful( http://www.ncbi.nlm.nih.gov/pubmed/19386033).

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #25 on: September 14, 2012, 08:41:36 PM »
I realize this thread is almost a year old but ....

::BUMP::

I got a new job this week and again had to undergo a TB test. I got the injection on Tuesday (September 11) and went in last night (Thursday September 13) to get it read. It was flat flat flat!! ;D

Totally negetive test results. So last year's musta been a false positive. 8)

 

Offline TexDaddy

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Re: False positive on a TB test?
« Reply #26 on: September 14, 2012, 08:43:45 PM »
Totally negetive test results. So last year's musta been a false positive. 8)
Even more convicing, you are not very sick or dead at this time.  ;)

Offline Oil Lady

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Re: False positive on a TB test?
« Reply #27 on: September 14, 2012, 08:45:55 PM »
Even more convicing, you are not very sick or dead at this time.  ;)

You mean this guy here??   >>>  :zombie:


 :rofl:

Offline FreeLancer

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Re: False positive on a TB test?
« Reply #28 on: September 14, 2012, 10:01:12 PM »
Totally negetive test results. So last year's musta been a false positive. 8)

What a relief! 

May this bode well for your new job.   :)


Offline Oil Lady

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Re: False positive on a TB test?
« Reply #29 on: February 13, 2015, 12:06:56 PM »
::UPDATE::

Well I am bumping this thread yet again after several years. :)

I have had easily a half dozen or more TB tests in the past 4 years of being a CNA now. And all of them (except, of course, for that one injection/implantation which I explained to the world up in the OP of this thread) have proved negative.

I even did yet another double-tap of two TB tests in a row during the past 3 weeks, and both proved negative. (And I'm not hacking up gobs of blood, nor am I dead!  :zombie: )

So yeah .... I obviously had one false positive in my life. So false positives do happen from time to time.



And on other matters ... I noticed the very high view-count of this here thread (over 16,000 views so far!). And so I went to Google just now and I plugged in the following 6 words in quotation marks:

"false positive on a tb test"

and the VERY FIRST SEARCH RESULT which gets returned for that exact Google search is this thread right here in TSP. :)

https://www.google.com/?gws_rd=ssl#q=%22false+positive+on+a+tb+test%22 

I guess I'm famous now.  ;D

And I also guess that false positives might happen enough to prompt LOTS of people around the English-speaking world to go to Google and search for those six exact words.  :o

.
« Last Edit: February 13, 2015, 12:13:41 PM by Oil Lady »