All,

At our facility we require a "post" amnio sample before we dispense Rhogam. This is for patients over 17 weeks. What does everyone else require? I can't find anything in the literature stating the sample has to be a "post" amnio sample.

Thanks!

What tests do you do on the 'post amnio' sample?

We do an ABO/Rh/ antibody screen and fetal screen.

In the UK, one of the most relevant documents is the BCSH guideline ‘Guidelines on the estimation of fetomaternal haemorrhage (September 2009 update)’. This identifies amniocentesis (amongst other things) as a potential sensitising event, i.e, may give rise to a FMH. After 20 weeks gestation it is possible that the size of a FMH may exceed the capacity of a ‘standard’ dose of prophylactic anti-D (RhoGam) to clear all fetal cells before they cause ‘sensitisation’ of the mother. To ensure a sufficient dose of prophylactic anti-D is given, a FMH estimation must be performed after any sensitising event (after 20 weeks, anyway) to determine if a significant, e.g. over 4ml, FMH has occurred and what dose of prophylactic anti-D is required to ‘cover’ this bleed.

So, I guess that the ABO/Rh will go some way to confirming that you have the right patient, by matching the current results to any existing results. It will also serve to identify the D type of a ‘new’ patient. The antibody screen will give you an idea as to whether anti-D is already present (no doubt there is a policy in place for dealing with determining if this is ‘immune’, or due to a previous administration of prophylactic anti-D. Finally, your fetal screen will give you an idea as to whether there has been a (significant) FMH and what dose of prophylactic anti-D you’ll need to give (although you do this from 17 weeks, rather than 20 weeks, I guess this is adding a safety factor).

Edited January 24, 201114 yr by John Eggington
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In the UK, one of the most relevant documents is the BCSH guideline ‘Guidelines on the estimation of fetomaternal haemorrhage (September 2009 update)’. This identifies amniocentesis (amongst other things) as a potential sensitising event, i.e, may give rise to a FMH. After 20 weeks gestation it is possible that the size of a FMH may exceed the capacity of a ‘standard’ dose of prophylactic anti-D (RhoGam) to clear all fetal cells before they cause ‘sensitisation’ of the mother. To ensure a sufficient dose of prophylactic anti-D is given, a FMH estimation must be performed after any sensitising event (after 20 weeks, anyway) to determine if a significant, e.g. over 4ml, FMH has occurred and what dose of prophylactic anti-D is required to ‘cover’ this bleed.

So, I guess that the ABO/Rh will go some way to confirming that you have the right patient, by matching the current results to any existing results. It will also serve to identify the D type of a ‘new’ patient. The antibody screen will give you an idea as to whether anti-D is already present (no doubt there is a policy in place for dealing with determining if this is ‘immune’, or due to a previous administration of prophylactic anti-D. Finally, your fetal screen will give you an idea as to whether there has been a (significant) FMH and what dose of prophylactic anti-D you’ll need to give (although you do this from 17 weeks, rather than 20 weeks, I guess this is adding a safety factor).

John,

This sensitizing event does not just apply to detection of anti D but also any allogeneic antibody; especial those most prominent during gestation. Therefore, the post specimen would be screened and worked up if positive, with potential future titering.

I was referring specifically to the administration of prophylactic anti-D, rather than considering other antibodies

Also, I think that using a 'post amnio' sample to look for antibodies that may have been stimulated by the 'amnio' may be a bit unreliable. It will have been taken very shortly after the event (to get a reliable FMH estimation), so a later sample is more likely to detect additional/new antibodies, as well as giving a better idea of whether existing antibodies have been 'boosted'.

We do not wait to test after delivery, so testing after an amnio makes sense. If the FMH from the amnio is larger than 15ml of RBC's, then the patient would require more than one syringe of Rh Immune Globulin. This is the same for post delivery, or even continual spotting during the pregnancy. This would be a rare occurrance but this is a disease of prevention.

Just out of curiosity. How long post amnio are you collecting the sample? It might take a little while for the fetal cells to circulate.