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Unstable Hemoglobin Test

Unstable hemoglobin (Hb) is a rare, congenital defect caused by amino acid substitutions in the structure of Hb. It's called "unstable" because of the ease with which the Hb decomposes. The presence of unstable Hb may lead to the formation of small masses called Heinz bodies, which accumulate on red blood cell membranes. Although Heinz bodies are usually removed by the spleen or liver, they may cause mild to severe hemolysis. Unstable Hb is best detected by precipitation tests (heat stability or isopropanol solubility).

Purpose

• To detect unstable Hb

Patient preparation

• Explain to the patient that this test is used to detect abnormal hemoglobin in the blood.
• 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.
• As necessary, withhold antimalarials, furazolidone (from infants), nitrofurantoin, phenacetin, procarbazine, sulfonamides before the text because these drugs may induce hemolysis. If these medications must be continued, note this on the laboratory slip.
• Inform the patient 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.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Resume administration of medications withheld before the test.

Precautions

• Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly.
• To avoid hemolysis, do not shake the tube vigorously.

Reference values

When no unstable Hb appears in the sample, the heat stability test result is negative; the isopropanol solubility test result is reported as stable.

Abnormal findings

A positive heat stability test result or unstable solubility test result, especially with hemolysis, strongly suggests the presence of unstable Hb.

Interfering factors

• Failure to fill the tube completely, to use the proper anticoagulant, or to adequately mix the sample and the anticoagulant
• Hemoconcentration due to prolonged tourniquet constriction
• Hemolysis due to 20 mg rough handling of the sample
• Hemolysis due to antimalarials, furazolidone (in infants), nitrofurantoin, phenacetin, procarbazine, sulfonamides (possible false-positive or unstable results)
• High levels of Hb F (possible false­positive isopropanol)
• Recent blood transfusion

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