Red Blood Cell Survival Time

Normally, red blood cells (RBCs) are destroyed only when they reach senility. However, in hemolytic diseases, RBCs of all ages are randomly destroyed, resulting in anemia. The RBC survival time test measures the survival time of circulating RBCs and detects sites of abnormal RBC sequestration and destruction.

Survival time is measured by labeling a random sample of RBCs with radioactive chromium-51 sodium chromate (51Cr). This labeled group of RBCs is then injected back into the patient. Serial blood samples measure the percent of labeled cells per unit volume over 3 to 4 weeks until 50% of the cells disappear (disappearance rate corresponds to destruction of a random cell population).

During the test period, a gamma camera scans the body for sites of abnormally high radioactivity, which indicate sites of excessive RBC sequestration and destruction. Other tests performed with the RBC survival time test may include spot-checks of the stool to detect GI blood loss; hematocrit; blood volume studies; and radionuclide iron uptake and clearance tests to aid differential diagnosis of anemia.

Purpose

• To help evaluate unexplained anemia, particularly hemolytic anemia.
• To identify sites of abnormal RBC sequestration and destruction

Patient preparation

• Explain to the patient that this test helps identify the cause of his anemia.
• Advise him that he needn't restrict food or fluids.
• Explain that the test involves labeling a blood sample with a radioactive substance and requires regular blood samples at 3-day intervals for 3 to 4 weeks. Tell him who will perform the testing and where it will take place.
• Tell him that he may experience slight discomfort from the needle punctures. Reassure him that collecting each sample takes less than 3 minutes and that the small amount of radioactive substance used is harmless.
• If stool collection is required to test for GI bleeding, teach the patient the proper collection technique.

Procedure and posttest care

• A 30-ml blood sample is drawn and mixed with 100 microcuries of 51Cr for an adult (less for a child).
• After an incubation period, the mixture is injected I.V. into the patient. A blood sample is drawn 30 minutes after injection to determine blood and RBC volumes.
• A 6-ml sample is collected in a green-top tube after 24 hours; followup samples are collected at 3-day intervals for 3 to 4 weeks. (Intervals between samples may vary, depending on the laboratory.)
• To avoid error nom physical decay of the 51Cr, each sample is measured with a scintillation well counter on the day it's drawn.
• Radioactivity per milliliter of RBCs is calculated and the values are plotted to determine mean RBC survival time. Simultaneous gamma camera scans of the precordium, sacrum, liver, and spleen detect radioactivity at sites of excess RBC sequestration. A hematocrit test is done on a small portion of each blood sample to check for blood loss.
• At the end of the study, a sample is drawn to compare ending blood and RBC volumes with beginning volumes.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• This test is contraindicated during pregnancy because it exposes the fetus to radiation.
• Because excess blood loss can invalidate test results, this test is usually ontraindicated for a patient with active bleeding or poor clotting function. However, if the test is necessary for a patient with poor clotting function, observe the venipuncture sites carefully for signs of hemorrhage.
• The patient should not receive blood transfusions during the test period and should not have blood samples drawn for other tests.

Normal Findings

Normal half-life for RBCs labeled with 51Cr is 25 to 35 days. Normal gamma camera scans reveal slight radioactivity in the spleen, liver, and sometimes the bone marrow.

Abnormal findings

Decreased RBC survival time indicates a hemolytic disease, such as chronic lymphocytic leukemia, congenital nonspherocytic hemolytic anemia, hemoglobin C disease, hereditary spherocytosis, idiopathic acquired hemolytic anemia, paroxysmal nocturnal hemoglobinuria, elliptocytosis, pernicious anemia, sickle cell anemia, sickle cell hemoglobin C disease, or hemolyticuremic syndrome. If hemolytic anemia is diagnosed, additional tests using cross-transfusion of labeled RBCs can determine whether anemia results from an intrinsic RBC defect or an extrinsic factor.

A gamma camera scan that detects a site of excess RBC sequestration provides direction for treatment. For example, abnormally high RBC sequestration in the spleen may require a splenectomy.

Interfering factors

• Dehydration, overhydration, or blood loss (possible invalidation of results due to changed circulating RBC volume)
• Blood transfusions during the test period (alters proportion of labeled RBCs to total RBCs)