The RBC survival time test has its greatest use in the evaluation of known or suspected hemolytic anemia and is also indicated when the cause for anemia is obscure (abnormal sequestration of RBCs in the spleen), to identify accessory spleens, and to determine abnormal RBC production or destruction. Typically, the normal RBC survives about 110–120 days in a normal healthy adult. Under normal circumstances, RBCs are eliminated due to the aging of the RBC (senescence) and random hemolysis.

Scanning of the spleen is often done as part of this test. The RBC survival test usually is ordered in conjunction with a blood volume determination and radionuclide iron uptake and clearance tests. When stool specimens are collected for 3 days, the test is often referred to as the “gastrointestinal blood loss test.”

1. Obtain a venous blood sample of 30 mL.

2. Ten to 30 minutes later, reinject the blood after being tagged with a radionuclide, ⁵¹Cr.

3. Blood samples are usually obtained on the first day; again after 24, 48, 72, and 96 hours; and then at weekly intervals for 3 weeks. Time may be shortened depending on the outcome of the test. After counting the specimens, the results are plotted, and the RBC survival time is calculated. Results are based on the fact that disappearance of radioactivity from the circulation corresponds to disappearance of the RBCs, thereby determining overall erythrocyte survival. As part of this procedure, a radioactive detector may be used over the spleen, sternum, and liver to assess the relative concentrations of radioactivity in these areas. This external counting helps to determine whether the spleen is taking part in excessive sequestration of RBCs as a causative factor in anemia.

4. In some instances, a 72-hour stool collection may be ordered to detect GI blood loss. Obtain special collection containers labeled for radiation hazard. At the end of each 24-hours collection period, the total stool is to be collected by the department of nuclear medicine. This test can be completed in 3 days.

5. See Chapter 1 guidelines for safe, effective, informed intratest care.

1. Shortened RBC survival may result from blood loss, hemolysis, or removal of RBCs by the spleen, as in:

   1. Chronic granulocytic leukemia

   2. Hemolytic anemia

   3. Hemoglobin C disease

   4. Hereditary spherocytosis

   5. Pernicious anemia

   6. Megaloblastic anemia of pregnancy

   7. Sickle cell anemia

   8. Uremia

2. Prolonged RBC survival time may result from an abnormality in RBC production, as in thalassemia minor, and false-negative results when transfusion is given during the procedure.

3. If hemolytic anemia is diagnosed, further studies are needed to establish whether the RBCs have intrinsic abnormalities or whether anemia results from immunologic effects of the patient’s plasma.

4. Results are normal in:

   1. Hemoglobin C trait

   2. Sickle cell trait

5. Half of the radioactivity in the plasma may not disappear for 7–8 hours.

Pretest Patient Care

1. Explain the purpose and procedure of the test. Emphasize that this test requires a minimum of 3 weeks of the patient’s time, with trips to the diagnostic facility for venipunctures.

2. If stool collection is required, advise the patient of the importance of saving all stool and that stool must be free of urine contamination.

3. Refer to standard NMI pretest precautions.

4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Dehydration or blood loss can affect the RBC volume.

2. Blood transfusion during the testing period.

Normal

Normal half-time for survival of ⁵¹Cr-labeled RBCs is approximately 25–35 days.

⁵¹Cr in stool: <3 mL/24 hr or <3 mL/d