We have a case that has me totally puzzled.  A woman was admitted to L&D for a fetal demise at 38 weeks.  She is D negative and was sent for a RhIG evaluation.  The baby's blood type was unknown and they were unable to get a decent sample from the fetus after delivery.  Since I learned my lesson from this forum that we cannot do a fetal screen (rosette test) if the baby's blood type is unknown, we gave it to the main lab for a Kleihauer-Betke.  It came back at ~10% fetal cells.  We treated the patient using this information, giving her LOTS of WinRho to save her from multiple injections of Rhogam.  Knowing that the fetal blood volume from this calculation seemed unlikely, we sent a specimen out for fetal bleed detection by flow cytometry.  Unfortunately, we did not get the results back until she had already received her WinRho.  The flow cytometry showed a bleed of 0.1% fetal cells.  I suspected that mom might have Hgb F in her circulation, but hemoglobin studies show normal results (mostly A1 and A2).  We have repeated the KB using different specimens and different KB kits and continue to get at least 3% fetal cells on the slide.  Any idea what else may cause false positive KB tests?

 

Pregnant lady has slightly raised HbF level in third trimester. It also depends upon the staining procedure and how we we count the cell. We use Molison formula. Sometimes it's difficult to differentiate lymphocyte and fetal cells. Which might be counted as fetal cells and give false positive results. This is why flow cytometry is more accurate.    But as far as I know when baby group is not available it is safer to give Rh IvIgG to prevent sensitisation.  

In all the years we have performed Kleihauer-Betke in our lab, I have never seen a false positive. (Our patients with Hemoglobinopathies  are always sent for flow)  This year we did see a patient at 26 week gestation, who came in with a hard blow to the stomach. She had a positive Kleihauer-Betke stain and was given 2 doses of RhoGam. She was also sent for ultra sound and the physician could not see any bleed. We sent the sample off to our reference lab for flow testing and it came back as 0.0% for fetal bleed. Our procedure states when a stain is positive, it is to be repeated by another tech. This procedure was followed and the tech's counts were within acceptable limits. Our pathologist reviewed both techs slides and agreed that the counts were correct.

I have not been able to explain this to the attending physician's satisfaction. This patient now has been flagged to send sample out for flow and not perform the KB stain in house after delivery. 

Any other ideas out there?

Do you think Patient might have persistent hereditary Feta haemoglobin ( HPFH)? 

 

For our patient, her hemoglobin studies were normal.

Perhaps it is not a false positive KB but rather a "false negative" flow?  One of the reasons for false negative KB is incompatibility between mom and baby.   If specimen for flow was allowed to sit for a while, could it be possible that baby cells were destroyed by mom's antibodies?  

I do like the idea of a false negative better. I am sure of what I saw and my techs are competent in staining and counting. The flow sample was set out with a TAT of two days. We will see if mom is a negative KB when she comes back for delivery.

We in our blood bank have sent away kleihauer positive samples for flow cytometry testing which has come back as negative as the technique looks for 'RhD Antigens' on the foetal red cells, in which case it is likely that the foetal cells are D negative and a negative flow result is returned.