Anti-Xa Testing in Pregnancy

As I get closer and closer to the point in my pregnancy where things started to go wrong with Jackson, I have been continuing my research on the use of Lovenox in pregnancy. 

This is not really an emotional post, but more along the lines of "informative" for those who read my stuff by have no idea what medicine I'm on or why.

For those who aren't familiar with Lovenox, it is a low molecular weight heparin that is injected into your fat--in my case the belly.  It is considered an anticoagulant (reduces the ability of blood to clot), but with a unique factor: it doesn't pass the placenta so it considered very safe for a developing baby since the medicine doesn't reach the baby.

The biggest problem I have run across as I've tried to research is that so many of these studies are incredibly small! When you have a study that only includes 30 people, it wouldn't seem that you can make very many conclusions, especially when there are thousands of women in the US who are on this medication at any given time.

As an example, just a couple of days ago a medical study on the use of a specific LMWH known as dalteparin (similar to Lovenox, and I have a friend who is on this because she is allergic to Lovenox) was printed in the Lancet. The study concludes that "dalteparin does not reduce the occurrence of venous thromboembolism (blood clots), pregnancy loss, or placenta-mediated pregnancy complications in pregnant women with thrombophilia at high risk of these complications and is associated with an increased risk of minor bleeding."

The problem? The study took over 12 years to perform but only had 292 participants.  Wait, What?!?  How in the world do you conclude ANYTHING with a study that took over a decade and still had fewer than 300 participants and each participant would have been in the study for less than a year?!? In addition, when you consider how many women take LMWHs through pregnancy and the postpartum period, why weren’t more women included in the study?

The is pure speculation, but since the study was open-label and randomized, the women who agreed to participate were assigned to either a group that would receive the medication or one that wouldn't. Each of these women would have known prior to the study that they were at risk for developing blood clots during pregnancy or having a placenta problem because that was a requirement of the study.  I can't imagine any woman who has experienced pregnancy loss would agree to this kind of study, so in that sense I guess 292 subjects is large . . . and of those in the study, 2 patients were moved from the control group to the medicated group. I would speculate that something happened during the pregnancy, such as a blood clot, to require them to be on the medication but the study doesn't say.

I would love to take a look at the records of three of the MFM practices I'm familiar with locally to compare the pregnancy outcomes of patients on various doses of Lovenox and their anti-Xa test results, if those were tested.  I am quite certain that a simple computer program that could scour the medical records for key words would make this very easy.  Just search for Lovenox, anti-Xa, the result, and then how the pregnancy ended (pre-term, IUGR, stillbirth, live birth, etc).

What this boils down to is that I believe it is important for any pregnant woman to have their anti-Xa levels tested during pregnancy, and I'm not the only one who thinks that.

Consider this study: Anti-factor Xa plasma levels in pregnant women receiving low molecular weight heparin thromboprophylaxis

The conclusion has what sounds like a dire warning: 

Serial anti-factor Xa assessment for dose adjustment should be considered for all pregnant women receiving low molecular weight heparin. (emphasis mine)

Never one to research just one side of an argument, here is another article: Is the monitoring of anti-Xa activity necessary in pregnant women undergoing thromboprophylaxis?

Consider for a moment that even this abstract says that the patient needs to have their anti-Xa levels in a specific range before frequent monitoring "may not be required".  I translate that to mean that the test needs to be run until the anti-Xa levels are within a specific range and then consider backing off on the monitoring.

Last Thursday I met with my OB.  I asked her a question about the Vitamin K shot that newborn receive, usually right after birth, and how those that have inherited clotting diseases may be affected, since Vitamin K is a key element in clotting. She didn’t know the answer, so I will ask my MFM tomorrow when I see her.

My first ever anti-Xa test results had also been faxed over to my OB, so she had those.  The listed range was 0.3-0.7 and my result was 0.83.  I was out of the range, but my score was circled with a great big “GOOD!!” by it.

At first I was really nervous, because I’m OUT OF RANGE!  But I’ve done further research and found this: Nobody tells you what the range means!

Yeah, so I went to my Lovenox support group on facebook and asked them.  Based on what their doctors told them, a value of 0 means that there is no Lovenox in your system at all and your blood will clot just as it should.

Not good for someone who is trying to prevent that.

The acceptable range seems to vary based on what website Google pulls up but there is something consistent: someone on a prophylactic dose (generally 40mg/once a day) has an acceptable range closer to 0 than someone who is on a therapeutic dose (used to treat active clots or in women where the prophylactic dose is insufficient based on history). 

The range for therapeutic seems to be roughly 0.5-1.0, with a variation of ±0.2, depending on the source.  In other words, the larger the number, the less your blood is clotting.

So for me to be on the outside range is actually a very good thing.  I am not so far beyond the range that I need to worry about excessive bleeding, but nor am I on the end of the spectrum where I would worry that my dose was insufficient.

Or at least, that was the case a little over 3 weeks ago when I had the test run.

I hope that when I see Dr. H tomorrow (MFM), that she will write another lab request so that I can get tested again.  At my preconception appointment she had indicated that somewhere around 11-12 weeks, I would move to doing the shots twice a day.

I am 12 weeks today, so it is likely time to make the change.  It was at 17 weeks that I had my last “All is going great” ultrasound with Jackson, so I’m getting a little antsy!!