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RH antibody test (likelihood of false positive??)


younglove
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Hi, I would like to know the likelihood of a false-positive RH anti-bodies test 16 weeks after Rhogam.

I found out I had a missed miscarriage at 12 weeks (baby stopped developing around 8 weeks) in October. I had a medically-induced miscarriage and received the Rhogam vaccine 8 hours after I started bleeding. This was my first pregnancy and I've never had a blood transfusion. I am O-, my husband is O+.

I had a prenatal antibody test 12 week after receiving Rhogam and tested positive. I had a follow-up test 4 weeks later (16 weeks receiving the vaccine) and again tested positive. Unfortunately they have not given me my titer results and have told me to come back in 4 weeks for another test. My doctor (I live in Canada and only have family doctor not an OBGYN) is unfamiliar with this situation.

I am worried that I have actually become sensitized. What are the chances that it is a false-positive due to the Rhogam vs. actually sensitized. All I have been told is that I tested "positive" for antibodies 16 weeks after the vaccine but the doctor said it could be a false positive. What is the average length of time before the RhIG no longer shows up on tests?

I am not pregnant now; we would like to know if I am actually sensitized before conceiving again. Thanks very much!!

Edited by younglove
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Depending on many variables, RhIG (RhoGam) antibody has been known to remain detectable for 6 months. It is frequently detectable after 12 weeks.

And sorry for your loss. Miscarriages can be hard and make the "mommy worries" even worse next time. Try not to worry too much. Remember, most of the things we worry about never actually happen.

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Hi younglove,

First of all, sorry to hear about the loss of your first pregnancy; that must have been very traumatic for both you and your husband.

To answer your questions, I'm afriad that I must start by saying that I am a British Biomedical Scientist, and am, therefore, not qualified to make a diagnosis. If I so did, I would lose my State Registration (quite correctly) and be thrown out of my job (again, quite correctly). So, you MUST not take anything I say here as the "Gospel Truth", but I will give you some answers.

To begin with, however, I am afriad that I must ask you some questions.

Do you know what dose of anti-D immunoglobulin you were given?

In what part of your body was the injection given? In the past, the anti-D was routinely given into the buttock muscle (the gluteal region) - this used to be known, tongue in cheek as "anti-D for mums' bums" - or the deltoid (shoulder) muscle? This is quite important to know, as the adsorption into the circulation is quite different, depending upon where the injection was given.

Once I know the answer to these ttwo questions, I may be able to give you a more educated answer. Sorry not to be able to go further until I know the answer to these two questions.

All that having been said, the answer is yes; it may be a "false positive" finding.

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Thank you both for the kind words and reassurance that there is a chance it could be a false positive.

I was given 300 mg of Rhogam

They injected it in my shoulder (I am a small girl - only 115 lbs if that helps).

Also, I just got my titering results. It was 1:1 with the first test and 1:2 with the second. I guess it is really bad that it went up! so sad...

I am so worried and disappointed :(

Thanks again for the assistance.

Edited by younglove
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Thank you both for the kind words and reassurance that there is a chance it could be a false positive.

I was given 300 mg of Rhogam

They injected it in my shoulder (I am a small girl - only 115 lbs if that helps).

Also, I just got my titering results. It was 1:1 with the first test and 1:2 with the second. I guess it is really bad that it went up! so sad...

I am so worried and disappointed :(

Thanks again for the assistance.

Well, don't be worried and disappointed.

The thing is that, at 12 weeks gestation, the foetus's (sorry, but that is the correct term - I KNOW it was your and your husband's baby) red blood cells will only just have started to express the Rh antigens. At that gestational age, the amount of red cells that could have escaped from the foetus's circulation into your own circulation (to stimulate your immune system to make anti-D (anti-Rh) would be very small, and it has been shown that stimulation of the immune system is, to a certain extent, governed by the amount of red cells that stimulate it in the first place. I would be simply amazed if, given the good treatment that you have very obviously been given, together with the follow-up screening that you are going through, if they had not done a test to find out accurately the volume of the foeto-maternal bleed (how much foetal blood had escaped into your circulation) and so had given you sufficient anti-D immunoglobulin to cover any such bleed (and we always give a "bit extra", just in case).

It is true that an injection given into the shoulder adsorbs into the circulation quicker than the one into the bum, but, there are two things here that have to be taken into account. Firstly, every lady is different; some adosrb the anti-D into their circulation quickly; some adsorb it at the "standard rate"; some adsorb it slowly. Secondly, as you are of small stature (115 lbs), the amount of anti-D given to you (the same as would be given to any lady in your situation) is, pound for pound, larger (if you see what I mean) and would, by definition, last longer in your circulation (anti-D has what is known as a half-life, as does any antibody, but if you start with a higher concentration [because you are small] then it will last longer). I'm not sure I've explained that as well as I could, but I know what I mean!!!!!!!!!!!!!!!

I would not worry two hoots about the titre results! Firstly, if one person performs a titration, it is not rare, but is unusual for them to get exactly the same results two times running, even using the same sample. It is just NOT that accurate a test. Secondly, if the same person does not erform both titres, it is even more unusual for them to get exactly the same results (humans are not all the same, and that goes for testing blood samples too). Thirdly, although your titre appearsI to have doubled, in actual fact, that means there is one tube difference in the titration, and that means nothing (see my comments immediately above about test accuracy and precision). If there was a difference of 1:1 to, say, 1:8, then I would be a bit more concerned, but there isn't! The difference between 1:1 and 1:2 could easily be (and probably is) experimental error (an acceptable error, I might add).

Mabel, who I regard as an expert, is absolutely correct in what she posted. DO NOT WORRY - it probably won't happen.

IF, and it really is a BIG if, the worst comes to the worst, and you have made your own anti-D, it is NOT the end of the world. In the UK, we measure anti-D slightly differently to the way it is measured in North America (as a continent, I mean - both the USA and Canada). It is not a better way; it is just a different way. In the UK, we measure anti-D as International Units per millilitre of plasma (IU/mL). Any anti-D level above 15 IU/mL gives us severe cause for concern. I have personal experience of pregnancies that I have followed where the anti-D level of the mother has reached well over 500 IU/mL, and the baby ahs been born perfectly healthy, and with no lasting problems whatsoever, because the Specialist Foetal Medicine Units are so good these days that they can transfuse the foetus in utero, and "salvage" (I loathe the term "salvage", but that, nevertheless, is the term used) babies these days that would nevr have survived about 50 years ago, AND THESE BABIES ARE PERFECTLY HEALTHY. How they do it, I don't know, but the fact remains that they do!

I hope that helps a little bit.

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First, thank you so much Malcolm and Mable for your posts. It has been incredible helpful to discuss this with people who know the subject. It was frustrating when my doctor was unable to tell me the probability of a false positive vs. actual sensitization.

Thank you so very much for providing the information and analysis. It gives me hope to carry on until the

Another point I wanted to add is that the baby only developed to 8 weeks (however, I only found out at my first scan at 12 weeks and which point a the miscarriage was medically triggered). I think this point is further in my favor. From the research I've done, there is only a theoretical risk from a 8-week old fetus; I could not find research discussing actual cases of sensitization that early on. At 8 weeks it is equivalent in size to a raspberry, although it potentially had an active heart and blood cells. But as you mentioned, the amount of red blood cells would have been very small.

Thanks also for clarifying/demystifying the titer results.

I have been referred to an OBGYN (who will hopefully have greater knowledge about this subject than my doctor) and I'll have another blood test in 3 weeks. I'm very hopeful that everything will come back clean. What result should I be looking for? Less than 1:1? Or negative?

My husband and I plan to start trying to conceive again in two months so hopefully we will know with certainty before than.

Again, I can't thank you enough for your kind words and the information you provided!! It has given me hope to carry on until the next test...

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Younglove,

Do not be concerned it the titre is still positive, as explained above sometimes a passively aquired anti-D can last for 6 months after the RhIG administration.

Even if you were to develop a true antibody you chances of having a healthy baby are great! The advances in obstrentics and neonatology are amazing and still happening. Women who would have never had the slightest chance of having a baby are now proud mothers.

I wish you and your husband the best of luck and a house full of bouncing babies..:redface:

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Just a note from personal experience. My wife, a nurse of course, has, amoung her other antibodies, anti-D. This developed due to the birth of our son. (Long story I won't burden you with here.) Four years later our daughter was born 6 weeks early and required a double exchange transfusion due to the anti-D. This little girl is now 27 with 2 kids of her own. Having anti-D is not nearly as devastating as it once was. Please keep us posted. You have our interest.

:comfort:

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Thank you everyone for responding! I appreciate your input!

I have done a bit of research just to understand everything a bit better. One woman online posted that she had false-positive tests for just over 6 months but eventually all subsequent tests were negative.

I'm still holding out hope that it is still a false positive! I'll try not to worry too much though. :)

Edited by younglove
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  • 3 weeks later...

I just spoke with my doctor. Antibody titers were at 1:1 after 19 1/2 weeks.

I also found out my doctor had misinformed me! The titer was 1:2 12 weeks after the vaccine and 1:1 16 weeks after the vaccine. I'm hoping this means it is even more likely that it is just a false positive...

Is 1:1 the lowest score? Is this what you would expect to find if it's a false positive after this long? Or could it be lower than 1:1 (e.g. barely detectable)? I'm wondering when I will know for sure. After 6 months / 26 weeks?

My doctor could not tell me anything about titering. She had referred me to a specialist but was told it could be months before I get an appointment.

Thanks!

Edited by younglove
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The titer of 2 at 12 weeks and 1 at 16 weeks could (probably) indicate a decreasing titer. A decrease in the titer after a RhIg (Rhogam) injection over time is what we normally see. When testing with the method we use here (gel) it is not unusual to have a pisitive antibody screen detecting the RhIg as long as 6 months after the injection. It sounds like things are progressing well (?) for you? Hope this helps a bit.

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1:1 is undiluted sample. Then we make doubling dilutions 1:2, 1:4, 1:8 etc. We usually need a reaction of at least a certain strength to say that a certain dilution is positive. Once the antibody gets too weak to react in the titration method the results will be "negative" or "<1" or similar wording. At 1:1 it is still reacting in the undiluted sample in the titration method so may still get weaker until undetectable by that method or even the antibody detection methods.

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Dear Subbusp. The answer depends on what your national guidelines require, but in general just tesing with one single anti-D is not a good idea. Just because an anti-D picks up DVI it doesnt mean it will pick up all partial Ds. It will however pick up all but the very weakest of weak Ds.

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YoungLove,

I wanted to first offer my sympathy for you loss and having read through these very informative posts I was moved to speak with some of my colleages about your case. One of my colleages, who has four children, said that she miscarried the first time as well. Collectively, they stated that they have all experienced this either individually or through someone they new, and that full-term pregnancies afterward produced healthy children. So I hope that this experience, as sad as it is, does not cause you to lose hope in delivering many children; it can, and has been done, so keep trying.

Best Wishes

Edited by rravkin@aol.com
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