Saturday, January 19, 2008

Daxor BVA-100: Turn Up The (Blood) Volume

No doubt some of you have performed blood volume analysis in the past, or at least had to learn how it was done. Remember the good old days, when we labelled red cells with chromium (Cr-51) and the plasma with I-125 albumin, then we had to do painstaking measurements with well-counters, and lots of hand-calculations? That's probably why the test was so expensive, and why it was rarely ordered.

Enter Daxor and the BVA-100, pictured here:

This takes most of the work (and guesswork) out of the test. Basically, the old method has been streamlined and simplified, but philosphically the method is similar. The basic principle involves dilution. If I have a bucket of water of unknown volume, and I dump a gram of sugar (or a microcurie of I-131) into it, I can then take a 1 cc sample of the water, and determine how much sugar (or I-131) is in that sample, and then work my way back to the original volume. If the sample contains 1 mg of sugar, I know that the volume must be 1 cc water x (1 gram sugar/1mg sugar) = 1000 cc water! Simple algebra, right? In the old days, we used this method to determine both plasma volume with I-125 labelled albumin, and red-cell mass with Cr-51. Graphically, think of it like this:

Daxor's method is a little easier from the user's standpoint:

The BVA-100 is a semi-automated blood volume analyzer. The instrument is used in conjunction with a single-use injection kit consisting of a precisely measured standard and matching injectate (of I-131 labelled human serum albumin). The injectate is packaged in a patented flow chamber designed to ensure 99.8%+ delivery. The kit improves accuracy and eliminates the many time-consuming and difficult steps required for on-site standard preparation.

The BVA-100 utilizes five separate sampling points taken at regular intervals starting approximately 12 minutes after injection. In effect, each sample measurement is a separate, single-point blood volume determination. The BVA-100 computes the transudation time for the tracer and calculates the true zero point blood volume with an accuracy of approximately +/- 2.5%. The BVA-100 provides interim blood volume results while the samples are being measured; preliminary data is available within 30 minutes and may be used to guide decisions in emergency situations.

Daxor's stroke of genius for me is the elimination of the direct labelling of the red cells, as well as plasma. Rather, a portion of the blood sample is reserved for hematocrit determination, and that ratio of red cells to plasma is then applied to the rest of the figures, i.e., the plasma volume. Simplistically, if the hematocrit is 50%, and the plasma volume is 2,000 ml, then the red cell volume has to be 2,000 ml as well. (The volume of white cells is negligible.)

The system goes even further, normalizing the readings to the patient's height and weight (and incidentally providing a reading of how far above or below average the patient might be.)

We all remember using blood volumes to document Polycythemia, an overabundance of red cells. As it turns out, blood volume measurement can be applied to a number of other situation such as:
    1. Acute blood loss during surgery or trauma
    2. Hypertension
    3. Syncope
    4. Orthostatic hypotension
    5. Congestive heart failure
    6. Septic shock and hypovolemia
    7. Anemia in cancer patients or HIV positive patients on chemotherapy Renal or kidney failure
    8. Preoperative screening for low blood volume
    9. Chronic fatigue syndrome
Have a look at this brief slide presentation from Daxor:

We purchased a BVA-100 over a year ago, and I am very pleased with the results. I am very slowly getting the word out to the clinicians as to just how much we can help them with this test, and they are actually starting to believe me! Fortunately, as the system uses I-131, it requires a nuclear license, and cannot be placed in the clinicians' offices. Sorry about that.

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