Estimation of FetoMateranl haemorrHage (FMH) (bsh 2009)
After 28 weeks, anti-D is still required even if RAADP given, but antibody screening is not.
Methods of FMH Estimation
Acid Elution (Kleihauer Test)
Based on HbF vs HbA
Best for initial quantification of FMH
- HbF is more resistant to alkali denaturation and acid elution than HbA
- Fixed, dry blood film placed in acid buffer, HbA is denatured and eluted leaving behind ghost cells. Cells containing HbF are stainable and stand out in a sea of maternal ghost cells.
- Thin, freshly prepared films easier to read (a 1:2 dilution may help)
- Modified test – elute only half the slide (allows comparison)
- Negative control (normal adult FBC)
- Positive control (cord blood added to adult whole blood in a 1:100 dilution
- A low power field using a x10 eyepiece and x10 objective will show a minimum of 1600 red cells (figure extrapolated from 100 cells seen with a x10 eye, x40 obj)
- 25 low power fields should be screened
- If 10 or more fetal cells seen then quantification must be performed
- Less than 10 cells can be considered
- Counted with aid of a Miller Square, counting minimum of 10,000 cells
- Mollinson formula then calculates the ml of FMH
Flow Cytometry
Fluorochrome conjugated with IgG monoclonal anti-D
Best as a reference test after positive acid elution finds >2ml FMH
- Mix sample thoroughly
- Wash cells to remove leukocytes and platelets
- Two samples should be tested in tandem; discrepancy suggests error in sample preparation or flow counting.
Reporting results to clinicians
Reports need to be:
Timely – within 72 hours
Clear – report result in ‘mL of fetal red cells’ rounded up to nearest 1ml
- Whether supplementary anti-D is required if a standard dose has already been administered.
- The Anti-D dose required to cover the reported bleed
- Advice on follow-up testing to ensure clearance of D+ cells
