Blood test for intrinsic factor. How accurate is it?

[Mavis15]

Have been well for quite a while on my eight weekly B12 injections but had been referred to a gastroenterologist for worsening IBS . He wants to find out why I have B12 deficiency and has requested I have blood test for intrinsic factor. Just wondering how accurate this is as sure I’ve had before and sure I’ve heard it’s not that accurate.

[wedgewood]

A scientist on the forum who has experience of the IFAB test said it was accurate . It just so happens that the antibodies that the test tries to trace come and go . You can have P.A. and the antibodies are just not present during the test . Our Chairman had to test 3 times before the antibodies were present . …

[mickeymouse42]

I noticed from your previous posts that you think that your low levels of B12 might be associated with IBS. Have you ever been tested for Bile Acid Malabsorption (BAM)? If your IBS involves diarrhoea, BAM is often the underlying cause.

BAM is associated with low levels of B12. BAM is tested via a SehCat scan which a gastroenterologist can authorise.

[Gambit62 Administrator]

The test is known to give false negatives 40-60% of the time.

This isn't because the test is inaccurate but because the test methodology means that it can pick up other metabolites so the range doesn't start at 0. Usually the range is given as something like <1, which is because readings below that range are likely to be other metabolites ... though they could also be IFAB. As a result the test is good in that a positive is unlikely to be false, so good confirmation of PA as a cause of B12 deficiency, but its a bit insensitive and can't be used to rule out PA as the cause of a B12 deficiency.

[Rexz]

The intrinsic factor antibody (IFAB) test accuracy can vary depending on several factors.

The IFAB test has a high specificity, meaning that if the test is positive, it is very likely that you have pernicious anemia. However, the sensitivity of the test is not as high, meaning that if the test is negative, it does not rule out pernicious anemia entirely. The reason for this is that the sensitivity of the test is purposely dialed down to eliminate most false positives.

You can also have false-positive or false-negative results on the IF antibody test. False-positive results may occur in people with other autoimmune conditions, such as lupus or rheumatoid arthritis. False-negative results may occur in people who have been treated with vitamin B12 supplements or have mild cases of pernicious anemia. A person with PA may test negative for IFAB then a year or so later, as the disease progresses, test positive. In other words most everyone with PA has IFAB but the test may not detect IFAB in earlier stages of disease progression.

Your gastroenterologist is right in wanting to confirm the PA diagnosis as PA is but a symptom of Autoimmune Metaplastic Atrophic Gastritis (AMAG). So "it ain't just B12" as I say. Another couple of tests to confirm this are Parietal Cell Antibody (PCAB) test and Serum Gastrin test. Although PCAB test is not a specific as IFAB it has a greater sensitivity thus by itself is not necessarily diagnostic for PA/AMAG. But should IFAB results come back negative and PCAB is positive along with a high serum Gastrin that would also be confirmatory for PA/AMAG. The reason for the high Gastrin is that when enough Parietal Cells are destroyed you will have low (hypochlorhydria) or no (achlorhydria) stomach acid. This low/no acid causes a whole host of digestive issues a couple of those being SIBO and IBS.

You may want to recommend to your Gastro to run the PCAB and Serum Gastrin along with your IFAB test.

Best wishes, Rexz

[Mavis15 in reply to Rexz]

Thank you for you very informative reply. Gastroenterologist said if was positive I would need regular gastroscopes. I will take your advice.

Best wishes

[Nackapan]

Picks up about 50% with PA .As wedgewood says can take several tests to get a positive result as antibodies tests are dependant on picking them up when they are there.

My surgery were testing all on b12 injections.

If serum b12 in range ( shouldn't be trsted)

Shoukd be above range!!

And negative for Intrinsic factor antibodies B12 injections were going to be stopped !!

Wrong !!

[Mavis15]

Thank you all for all your replies. Very useful information. Greatly appreciated. This site is wonderful.