HEMATOLOGY: Question 1

What is the screening test for sickle cell disease?

ANS:  SOLUBILITY TEST

PRINCIPLE: 

Hemoglobin S is less soluble in a reducing agent than other forms of hemoglobin.

NOTES:

The solubility test is the most common screening test for sickle cell or presence of HbS. It is based on the relative insolubility of HbS when combined with a reducing agent such as sodium dithionite. When anticoagulated blood is mixed the reducing agent, the red cells will lyse due to the presence of saponin and the hemoglobin in the red cells will be released. 

The solubility test cannot be used to differentiate sickle cell disease (homozygous for HbS) from sickle cell trait (heterozygous for HbS). 

Hemoglobin electrophoresis is considered the diagnostic procedure and is especially important considering that there are other hemoglobin variants that will also cause a positive solubility test, such as HbC Harlem. 

 If HbS is present, it will form liquid crystals and give a cloudy or turbid appearance to the solution. If HbS is not present, the solution will appear transparent. 

SOURCES OF ERROR: 

1. A patient with an exceptionally high hematocrit may give a false positive result, while an individual with a very low hemoglobin may give a falsely negative result.
2. Unstable hemoglobins may give a false positive result. 
3. False positives can occur with elevated plasma proteins and lipids.

source: shp.utmb.edu