April 3, 2002 --
Using isotope dilution methods, plasma and red cell volume determinations may be made. The most common clinical application of this test is when evaluating patients for polycythemia vera. Plasma volume determinations may be useful in select patients when more standard methods of estimating volume status are difficult, such as in burn patients. Other uses include evaluation of SIADH and primary hyperaldosteronism.
By diluting a known activity and volume of radiotracer in an unknown volume and then measuring the activity in a volume withdrawn after adequate mixing, one can determine the unknown volume. This relationship can be expressed by the following formula:
V1 * C1 = V2 * C2; therefore V2= V1* C1/ C2
V2= unknown volume
V1 = volume of tracer injected
C1 = Concentration of injected isotope (counts/min/ml)
C2= Concentration of sample obtained after adequate mixing (counts/min/ml)
For this technique to give accurate results several assumptions must be met:
(Diagnostic Nuclear Medicine. Third Edition, Vol 2.)
For plasma volume determination, I-125 RISA is used (T1/2=60.2days, gamma [6.6%]=35.5KeV, characteristic x-ray [76%]=27KeV).
Net std. cts. x Dilution factor (1000) x Vol. injected Plasma volume (cc's) = ------------------------------------------------------ Net plasma counts (Or use extrapolated counts to T0)
At this point, total blood volume and RBC volume can be calculated from the plasma volume, however, this is not recommended.
Total blood volume = Plasma volume (CC) / [1- (Hct x 0.86)] RBC volume = Total blood volume - Plasma volume
The above calculations rely on a measured peripheral hematocrit multiplied by an f-cell ratio (body hematocrit divided by venous hematocrit). Since the hematocrit in peripheral blood is higher than central blood a correction must be made. The mean f-cell ratio is .89-.92 in normal patients, however, various conditions can alter this ratio, such as hospitalized and anemic patients (mean f-cell ratio = .86). Patients with splenomegaly may have ratios >1.0 . To prevent the introduction of this error in blood volume measurements, it is recommended to perform plasma and RBC volume determinations separately.
Cr-51 (T1/2=27.8 days, gamma=324 Kev) is the label most commonly used. As chromate, the label enters RBC's and binds to the beta chain of hemoglobin. Tc-99m can also be used, however, its rate of elution from RBC's is much faster than for Cr-51 (1% per day in normal patients and slightly higher in patients with hemoglobinopathies) and its use is limited to studies with short sampling times.
[(WBi x (25/3)) - (PLi x PCTi)] x (Wt. Injected / Sp. Gravity) x HCTi RBC Vol. = --------------------------------------------------------------------- WBs - (PLs x PCTs)
Great care must be taken >when collecting and preparing samples as significant errors can occur. The most common involve inaccuracies in syringe calibration. Errors in pipetting, geometric counting and loss of radiolabeled albumin onto glassware are also possible considerations.
The volumes obtained are divided by the patients weight to give results in cc/kg. Blood volumes are closely related to lean body mass (muscle requires a larger blood volume than fat), thus the normal range of volumes varies within a population. The normal range also assumes a patient is within their ideal body weight and for patients who do not fit this description an adjusted weight can be used instead of their actual weight. *For obese patients, 20% of the amount over their ideal weight is added to the ideal body weight. For extremely fit patients with a low proportion of body fat, 10-15% of their actual weight is added to the measured weight. Adjustments can also be made for patients with amputations.
[Diagnostic Nuclear Medicine. Third Edition, Vol 2. ]
|Plasma Vol.||RBC Vol.||Tot. Blood Vol.|
Values are in cc/kg and based on adjusted ideal weight.
Evaluation of an elevated hematocrit is a common reason for performing blood volume measurements. By determining the plasma volume and the RBC volume, one can distinguish relative from true polycythemia. If additional information is taken into account such as a patients CBC, erythropoetin levels and physical exam, it is then possible to determine the exact cause for an elevated HCT as shown below:
|Relative polycythemia||Polycythemia Vera||Secondary polycythemia|
|WBC / PLT count||Nl||Inc.||Inc.|
|Erythropoetin||Nl||Nl or low||Inc.|
[Stein. Internal Medicine. 4th Edition, 1994]
In relative polycythemia, the plasma volume is contracted causing an elevated HCT even though the RBC mass is normal. Conditions which could cause this include: Diuretics, burns, volume loss (emesis / diarrhea), and stress (Gaisbock's syndrome).
Polycythemia vera is an abnormal clonal proliferation of red cells, growing without
influence from erythropoetin. Actually, all hematopoietic precursor cells may be affected;
thus, elevation of white cell and platelets may be noted as well. The disease carries a
poor prognosis (median survival 1-1/2 years) if not treated, usually due to thrombotic
complications. Diagnosis is made by the following criteria:
|Increased RBC mass||Increased Plt count > 400k|
|Arterial O2 saturation >91%||Increased WBC count >12k|
|Splenomegaly||LAP score >100|
|Serum B12 > 900 pg/ml|
[Merck Manual of Diagnosis and Therapy. 16th Edition, 1992. ]
Secondary polycythemia causes elevation of the RBC mass alone, in response to hypoxia and/or erythropoetin stimulation. Causes of secondary polycythemia should be excluded before performing blood volume measurements and this involves evaluating a CBC and arterial blood gas/ pulse oximetry.
Several causes of secondary polycythemia are listed below: