Hemoglobinopathies: A Useful Algorithm
by Beth A. Pletcher, MD, September 1998

Hemoglobin screening poses a problem for many obstetricians because of the nuances of testing and various hemoglobin variants found in different populations. Here is just one possible way to approach screening.

All black and Hispanic patients should have a hemoglobin electrophoresis and CBC with MCV to look for S trait, C trait or microcytosis that might be a clue to thalassemia trait. Patients of Mediterranean origin (Italy, Greece) or Asian background should have a CBC with MCV.

If a patient has microcytosis (MCV <80), additional testing should be pursued.

The Mentzer Index is sometimes helpful in differentiating thal trait from iron deficiency as is the RDW.

Mentzer Index=MCV / RBC

if <13

suggests possible thal trait

if >14

suggests possible iron deficiency

RDW

if <19

possible thal trait

if >19

possible iron deficiency

Determine MCV

if low

do quantitative HGB A2, HGB F, ferritin and lead, FEP and TIBC

if normal

no more studies

HGB A2 NL=<3.6% HGB F NL=<2.5%

Differential diagnoses

A2

F

ferritin

TIBC

MCV

Iron deficiency

nl

nl

low

high

low

Chronic anemia

nl

nl

low

nl

low

b- thal trait

high A2

+/- F

nl

nl

low

thal trait

nl

nl

nl

nl

low

Definitive diagnosis of these hemoglobin variants can be accomplished using molecular diagnostic techniques.

Although many practitioners recognize the risks for sickle cell trait in the black population and risks for thalassemia minor in the Mediterranean population, it is important to screen Hispanic patients for both and black patients for thal minor since these are quite prevalent in these populations as well. If a pregnant women screens positive for one of these variants she needs to have counseling regarding this finding and when at all possible her partner needs to be screened. For couples where both partners are found to carry a hemoglobin variant, genetic counseling and prenatal diagnosis are available if they so choose.