The purpose of the total blood volume test is to assess the volume of total blood, plasma, and RBCs. It is performed to determine circulating blood volume, to help evaluate the bleeding or debilitated patient, and to determine the origin of hypotension in the presence of anuria or oliguria when dehydration may be the cause. This determination is one way to monitor blood loss during surgery; it is used as a guide in replacement therapy after blood or body fluid loss and in the determination of whole-body hematocrit. The results are useful in choosing the most appropriate blood component for replacement therapy (whole blood, plasma, or packed RBCs).

Total blood volume determinations are of value in the following situations:

1. To evaluate GI and uterine bleeding

2. To aid in the diagnosis of hypovolemic shock

3. To aid in the diagnosis of polycythemia vera

4. To determine the required blood component for replacement therapy, as in persons undergoing surgery

This test reveals an increased or decreased volume of RBC mass. A sample of the patient’s blood is mixed with a radioactive substance, incubated at room temperature, and reinjected. Another blood sample is obtained 15 minutes later. The most commonly used tracers in blood volume determinations are serum albumin tagged with ¹³¹I or ¹²⁵I and patient or donor RBCs tagged with chromium 51 (⁵¹Cr). The combination of procedures (total blood volume) is the only true blood volume. Other volume studies are plasma volume and RBC volume, which may be done separately. Serum albumin tagged with ¹³¹I or ¹²⁵I may not always be available from the manufacturer.

The plasma volume is used to establish a vascular baseline, to determine changes in plasma volume before and after surgery, and to evaluate fluid and blood replacement in patients with GI bleeding, burns, or trauma. The ⁵¹Cr RBC volume study is done to see what percentage of the circulating blood is composed of RBCs. This procedure is performed in connection with evaluation of RBC survival or GI blood loss and in ferrokinetic (turnover or clearance rate of iron in the body) studies. These tests can be done simultaneously.

1. Record the patient’s height and current weight.

2. Obtain venous blood samples and mix one blood sample with a radiopharmaceutical agent.

3. Fifteen to 30 minutes later, reinject the blood radionuclide.

4. About 15 minutes later, obtain another venous blood sample and have it examined in the laboratory.

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

1. A normal total blood volume with a decreased RBC content indicates the need for a transfusion of packed RBCs.

2. Polycythemia vera may be differentiated from secondary polycythemia.

   1. Increased total blood volume due to an increased RBC mass suggests polycythemia vera. The plasma volume most often is normal.

   2. Normal or decreased total blood volume due to a decreased plasma volume suggests secondary polycythemia. The RBC most often is normal.

Pretest Patient Care

1. Explain the purpose, procedure, benefits, and risks of the test. Tell the patient that blood samples and IV injection are part of this test and that no imaging takes place.

2. Weigh the patient just before the test if possible.

3. Refer to standard NMI pretest precautions.

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

Normal

Total blood volume: 55–80 mL/kg or 0.055–0.080 L/kg

RBC (erythrocyte) volume: 20–35 mL/kg or 0.020–0.035 L/kg (greater in men than in women)

Plasma volume: 30–45 mL/kg or 0.030–0.045 L/kg