Full history/Diagnosis is available below
4.2 uCi I-125 human serum albumin, IV.
6 drops SSKI.
The results (summarized in bold) are the most important for intepretation. The remainder demonstrates the calculations used in determining the result and duplicate crosschecks for ensuring validity.
Corrected hematocrit (peripheral): 10 min sample): 69%
RBC mass (ml/kg): Measured, (10 min): 73.1 (low normal: 15.2, mean: 20.3, high normal: 25.3)
Plasma vol (ml/kg): Measured, (10 min): 44.4 (low nl: 24.8, mean: 33.0, high nl: 41.3)
Whole-body hematocrit: (10 min) 62.2
Whole-body / venous hematocrit (F-cell ratio): (10 min) 0.90
Numbers used for calculations follow; you may skip to the discussion below if desired.
Blood volume examination (12/3/97):
63 yo woman, Weight: 83 Kg, BSA (sq m): 1.97
Corrected hematocrit (peripheral):
10 min sample: 69%
20 min sample: 69%
Cr-51 syringe weights (gm):
Full: 21.43
After Std: 20.32
Empty: 9.65
I-125 syringe weights (gm):
Full: 14.572
After Std: 14.12
Empty: 9.62
Counting data (all samples 2 ml):
Cr-51 Std, Cr-51 window, number 1: 45723
Cr-51 Std, Cr-51 window, number 2: 46097
Cr-51 Std, I-125 window, number 1: 782
Cr-51 Std, I-125 window, number 2: 796
I-125 Std, Cr-51 window, number 1: 524
I-125 Std, Cr-51 window, number 1: 525
I-125 Std, I-125 window, number 1: 47510
I-125 Std, I-125 window, number 2: 47314
10 min sample, Cr-51 window, number 1: 5492
10 min sample, Cr-51 window, number 2: 5395
10 min sample, I-125 window, number 1: 4210
10 min sample, I-125 window, number 1: 4141
20-min sample, Cr-51 window, number 1: 5476
20-min sample, Cr-51 window, number 2: 5446
20-min sample, I-125 window, number 1: 4206
20-min sample, Cr-51 window, number 2: 4127
Blood Bkg, Cr-51 window, number 1: 520
Blood Bkg, Cr-51 window, number 2: 491
Blood Bkg, I-125 window, number 1: 120
Blood Bkg, I-125 window, number 2: 103
Air Bkg, Cr-51 window, number 1: 501
Air Bkg, Cr-51 window, number 2: 503
Air Bkg, I-125 window, number 1: 127
Air Bkg, I-125 window, number 2: 101
Crossover factor: 0.0149
RBC mass (ml/kg):
Measured, 10 min: 73.1 (low normal: 15.2, mean: 20.3, high normal: 25.3)
Measured, 20 min: 72.8
Plasma vol (ml/kg)
Measured, 10 min: 44.4 (low nl: 24.8, mean: 33.0, high nl: 41.3)
Measured, 20 min: 44.5
Total vol of whole blood
10 min 117.5 (Mean: 53.3)
20 min 117.4
Whole-body hematocrit
10 min 62.2
20 min 62.1
Whole-body / venous hematocrit (F-cell ratio):
10 min 0.90
20 min 0.90
The fact that two different isotopes are being measured simultaneously creates minor measurement difficulties. Two different energy windows are set for the two isotopes, but there is some crossover of one isotope into the energy window of the other. The gamma photon energy of Cr-51 is much higher than that of I-125. Therefore, downscatter from Cr-51 will fall into the I-125 window. The degree to which this occurs can be determined by measuring the activity in the I-125 window from a Cr-51 standard of known activity. A correction cross-over factor can then be determined to compensate for this scatter. The Cr-51 activity in the blood samples is multiplied by the cross-over factor and this is then subtracted off from the measurement in the I-125 window.
The blood volume examination is complex and there are therefore many opportunities for errors and artifacts. At our institution, each sample and standard are run in duplicate to help prevent this. Contamination with another radiopharmaceutical (particularly Tc-99m) is is easily introduced (just a tiny amount on a glove that is inadvertently introduced into a sample can cause a dramatic artifactually elevated count rate). If a sample is suspected to have a spurious high count rate, the vial may be recounted after a delay (for example, 1 or 2 days later). The physical decay rate can then be used to determine if contamination was present (both Cr-51 and I-125 have much longer half-lives than Tc-99m).
If there is extravasation at the injection site, the concentration of tracer in the blood will be reduced, making the apparent volume of distribution and therefore the measured red cell mass or plasma volume appear larger than it really is. Conversely, if the measured concentration is too high, the measured red cell mass or plasma volume will be artifactually reduced. It is therefore important to inject and draw subsequent samples from different veins (preferably from different arms) to avoid residua in the line or vein from artifactually increasing the measured concentration.
The whole-body hematocrit is lower than the peripheral hematocrit. This is due to streaming effects in which plasma flows more readily through capillaries than red cells. Thus, there is relatively a higher proportion of plasma in the capillaries than in larger, peripheral vessels. The F-cell ratio is a measure of whole-body hematocrit, determined in the blood volume examination, to the measured peripheral hematocrit. This number is typically in the range of 0.9.
Albumin diffuses slowly into the extracellular space. 10% will leave the intravascular space at one hour. The error is 1 to 3% normally at 10 minutes. This error is increased if there is protein loss or if vascular permeability is increased (e.g., burn patients). One can approximately compensate for the error by obtaining two measurements (e.g., at 10 and 20 minutes). The later, 20 minute sample, will show an increase in volume of distribution and therefore an increased measured plasma volume. The correct value can be approximated by extapolating from the 20 minute measurement through the 10 minute measurement back to time 0. In practice, the degree of error is small and this extrapolation is not generally necessary.
Polycythemia vera was first described by Vaquez in 1892 as characterized by a triad of cyanosis, polycythemia and splenomegaly. It is currently believed to result from the transformation of a single stem cell by analysis of the polymorphic X-chromosome marker glucose-6-phosphate dehydrogenase. The onset is insidious and is most common in elderly patients. Symptoms include headaches, plethora, prurutis, thrombosis and GI bleeding. Thrombosis and hemorrhage are the most common complications (seen in 1/3) and manifesting in such conditions as CVA, MI, DVT / PE and Budd-Chiari. There is commonly a low partial pressure of oxygen. An absolute neutrophilia is seen in 2/3. Elevated platelets are seen in 1/2 (> 1,000,000 in 10%). The platelets do not function properly. PT and PTT are generally normal.
Diagnostic features include erythrocytosis, leukocytosis, thrombocytosis and splenomegaly. The phases of polycythemia include the plethoric phase (increased red cell mass), followed after years by the spent phase characterized by anemia and marrow fibrosis (similar to myelofibrosis).
Treatments have included phlebotomy, P-32 radiation treatment, chlorambucil and hydroxyurea. The polycythemia study group (1986) determined that the median survival for phlebotomy alone is 13.9 years, P-32 11.8 years and chlorambucil 8.9 years. Some have reported an increased incidence of leukemia with P-32 or chlorambucil therapy.
Currently, phlebotomy is considered first-line therapy. If the hematocrit is > 64%, phlebotomy is generally conducted every other day until the value drops to 55%. It is then performed as needed to maintain hematocrit at 45%. Hydroxyurea is a second-line treatment that is useful if the platelet count is greater than 1,000,000.
References and General Discussion of Blood Volume (Anatomic field:Vascular and Lymphatic Systems, Category:Neoplasm, Neoplastic-like condition)
Return to the Teaching File home page.