all tests have less than 100% accuracy
the accuracy of a test is usually measured in terms of the following 2 numbers
- sensitivity ( % chance that the test correctly identifies a disease is present - when it truly is)
- specificity ( % chance that the test correctly identifies people without the disease)
and the closely related measures
-True positive: the person has the disease and the test is positive.
-True negative: the person does not have the disease and the test is negative.
-False positive: the person does not have the disease and the test is positive.
-False negative: the person has the disease and the test is negative.
this diagram illustrates how these results may be typically found in a tested population - and shows how - very often tests are imperfect - and a cut off value is then selected above which results are given as positive - and below which results are given as negative - and there is therefore some results that fall in the overlap zone - false positives and false negatives
VIEW IMAGEi think what you are asking is
"What is the % chance that an auto-immune disease i know i do in fact have - has caused a PCR test for a bacterial infection eg bart to be positive - when the infection is not in fact present" ( let me know if i have your question right)
so here we are interested in the false positive rate for PCR testing for bartonella - in other words the specificity of PCR testing for bartonella
here, unlike many older test technologies, like serology (antibody testing) PCR is actually highly specific (has almost no tail on the negative curve crossing the cutoff line in the above diagram)
it works by looking at a region of the bacterial DNA that is around 1600 base pairs long and is known to be highly variable from one type of bacterial to another - so it effectively provides a unique fingerprint - or 1600 digit long binary code for each genus of bacteria - so has a v high specificity % - in fact its so specific that its specificity is often given as effectively 100%.
Auto-immune diseases do not encode DNA - so no mechanism exists that would cause them to create a false positive signal on a PCR test.
for example, this study across a range of culture negative bacterial infections in human patients found 100% specificity
https://academic.oup.com/cid/article/53/12/1245/399923so the long answer is that its extremely unlikely - in the order of 0% chance that PCR tests would report positive for bacterial infection - when no infection was present - even in the presence of auto-immune disease.
note sensitivity can still be an issue for PCR based tests and was still only 40% in the above study - but this results in false negatives - rather than false positives - so PCR tests for TBD are still not perfect - and may still report negative results when in fact an infection is present. But if you get a positive results by PCR there is almost no chance its a false positive.
hope this helps
Post Edited (Garzie) : 10/5/2022 5:59:13 AM (GMT-7)