sickle screen; hemoglobin solubility

Introduction

Also see HemoTypeSC

Indications

• screening for
  • sickle cell trait &
  • sickle cell disease
  • other hemoglobin S genetic variants

Principle

• sodium dithionite is added to lysed erythrocytes to reduce hemoglobin
• the solution becomes turbid when hemoglobin S is present
• with other hemoglobins, the solution remains clear

Reduced Hgb S is insoluble in concentrated phosphate buffers. Under the same conditions, Hgb A, as well as most other hemoglobins, is soluble. RBC's are lysed by a surfactant & the released hemoglobin reduced by sodium hydrosulfite (NaHSO3) in concentrated phosphate buffer. The released Hgb S when reduced in the phosphate buffer forms a turbid suspension which is easily visualized. Hgb A, as well as most other hemoglobins, remain soluble under these conditions. Both sickle cell disease & sickle cell trait yield a positive result. Severe anemia can cause false negatives. Therefore, if the Hgb is < 8 g/dL, the sample volume for testing should be doubled. Elevated levels of Hgb F may result in false negatives. Thus specimens from infants < 6 months of age do not yield reliable results. Some rare hemoglobin variants, such as Hgb C Harlem, or C Georgetown may also give a positive sickle screen. Blood from patients with multiple myeloma, cryoglobulinemia, & other dysglobulinemias, may give false positives. This problem can be eleviated by saline washing the patient's red cells.

Clinical significance

• useful for screening
• does not detect
  • carriers of hemoglobin C
  • beta-thalassema
  • does not distinguish between sickle cell trait & sickle cell disease or other hemoglobin S genetic variants (hemoglobin SC)

Interferences

• false positives
  • lipemic specimens
  • abnormal gamma-globulins
  • polycythemia
  • increased numbers of Heinz bodies (post-splenectomy)
  • increased numbers of nucleated erythrocytes
• false negatives
  • blood hemoglobin < 7 g/dL
  • phenothiazines
  • high hemoglobin F (thus unreliable for newborns)
  • low % of hemoglobin S

More general terms

• clinical hematology test
• turbidometry

Additional terms

• hemoglobin S
• HemoTypeSC
• sickle cell (hemoglobin SS) disease
• sickle cell trait; hemoglobin S trait