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Home :: Sickle cell

Sickle Cell Test

The sickle cell test, also known as the hemoglobin (Hb) S test, is used to detect sickle cells, which are severely de­formed, rigid erythrocytes that may slow blood flow. Sickle cell trait (characterized by heterozygous Hb S) is found almost exclusively in people of African ancestry. It's present in nearly 8% of African Americans.

Although this test is useful as a rapid screening procedure, it may produce erroneous results. Hb electrophoresis should be performed to confirm the diagnosis if sickle cell disease is strongly suspected.

Purpose

  • To identify sickle cell disease and sickle cell trait.

Patient preparation

  • Explain to the patient that this test is used to detect sickle cell disease.
  • Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
  • Reassure him that drawing a blood sample will take less than 3 minutes.
  • Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
  • If the patient is an infant or child, explain to his parents that a small amount of blood will be taken from the finger or earlobe.
  • Check the patient's history for a blood transfusion within the past 3 months.
  • Inform him that food or fluids need not be restricted before the test

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 7-mllavender-top tube.
  • For very young children, collect capillary blood in a microcollection device.
  • If a hematoma develops at the venipuncture site, apply warm soaks.
Precautions
  • Completely fill the collection tube and invert it gently several times to thoroughly mix the sample and the anticoagulant.
  • Do not shake the tube vigorously.

Reference values

Results of this test are reported as positive or negative. A negative result suggests the absence of Hb S.

Abnormal findings

A positive result may indicate the presence of sickle cells, but Hb electrophoresis is needed to further diagnose the sickling tendency of cells. Rarely, in the absence of Hb S, other abnormal Hb may cause sickling.

Interfering factors

  • Failure to fill the tube completely, to use the proper anticoagulant in the collection tube, or to adequately mix the sample and the anticoagulant.
  • Hemolysis due to rough handling of the sample.
  • Hb concentration < 10%, elevated Hb S levels in infants under age 6 months, or transfusion within 3 months of the test (possible false-negative)

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