If you read this post about hemolytic disease of the newborn, you already know the answer: it’s used for determining the amount of fetal blood that has backed up into the mom’s circulation.

It’s usually done for the purpose of determining Rhogam dose. You need to make sure you give enough Rhogam to suppress the mom’s immune response. If there has been a little bleed, you give a little; if there has been a big bleed, you need to give more. Take a look at this chart if you want to know exact doses.

Here’s how it’s done:

1. Prepare blood smear from mom’s blood.

2. Expose blood smear to acid bath (this removes adult hemoglobin, which is acid-sensitive) but not fetal hemoglobin.

3. Stain smear. Fetal cells appear dark pink; maternal cells look like “ghosts.” Here’s what this looks like:

4. Count lots of cells and report percentage of cells that are fetal (specifically: you count the number of fetal blood cells per 50 low power fields. If you see 5 cells per 50 low power fields, that’s equivalent to a 0.5 mL fetomaternal hemorrhage).

If you want to get really fancy, you can look for fetal blood cells using flow cytometry. Using a sample of mom’s blood, apply an anti-HbF (fetal hemoglobin) antibody, and then run the sample through the flow cytometer. In the little printout, look for cells that stain intensely with HbF: these are baby’s cells! A few of mom’s cells will have weak HbF staining – this is normal in adults.

Top image credit: adamr.stone (http://www.flickr.com/photos/adamrstone/3098924060/) via cc license.