The tech note on your website states that the crit-line monitor is calibrated for the coulter counter with a mean cell volume of 91. My question, what if my pt has a mean cell volume of 100 or 107, (which I currently have), how does this effect the value on the monitor? Is it less accurate or is the lab value from spectra closer to the correct pt value? I like to use the crit-line to dose my epo, I need to know which value is more appropriate for my pt. I appreciate any info anyone has on this matter.

There are always going to be a few patients who are above or below the standard. The Hct. for this patient should not be affected by the MCV. You can definitely use the crit-line. It is calibrated to plus or minus 1 hct.

Dear RNCNN,
I would like to take this opportunity to readdress your question regarding MCV. As I stated previously the Crit-Lines accuracy standard is 1 SD= 1 Hct. This standard is true if the mean cell volume (MCV) of the red blood cell is 91 femtoliters. In Tech Note #11, the Crit-Line Hematocrit Accuracy, there is a formula that can be used to correlate the Crit-Line and lab measurements if MCV is less than 91.
If the MCV of the cell is larger, the hct artificially increases; same number of red cells, they are just larger and make the hct appear to be greater. The increase in MCV can be the result of changes in the osmolality and sodium levels when the blood is removed from the body and placed in a test tube containing other substances such as citrate or EDTA.
If you are interested in the formula and the math, please let me know.
The MCV standard of 91 was chosen because it was the average MCV of samples taken during several clinical studies done by In-Line Diagnostics Corp. when the intradialytic blood sodium concentration was between 132-137.
In the vast majority of patient samples no correction is necessary.
I hope this helps. I apologize for any confusion I may have caused. Thanks for your great questions. Please keep them coming!
Nancy

Nancy, Thanks for the info. I would like to know the math formula, as I have two pts who are not at 91 MCV

Dear RNCNN,
Here it is assuming the MCV is 107.
Hct Correction= 51.8498-0.915287(MCV)^2
51.8498-0.915287(107)+0.0037812(107)^2
51.8498-97.93571+43.29096
-2.8
This correction factor would then be added(subtracted in this case) to the Coulter counter Hct. Therefore if the lab returned a hct. of 39, subtract 2.8 and the Hct. to use as a comparison to the Crit-Line would be 36.2

Nancy, Thanks for the data, this will be most helpful to the docs

Hello CLM users. The correction equation for MCV given by Nancy is correct for CLM IIR users. CLM III users have a different correction factor. The definition of HCT is as follows:

HCT = [# of Red Blood Cells X Mean Cell Volume]/[Total Volume of the Sample]

If we take a sample and measure it with both the CLM III and the laboratory equipment (Coulter Counter is our standard), then here is how the math breaks out:

For the CLM III;
HCT(CLM) = [# of Red Blood Cells X 91]/[Total Volume of the Sample]

For the lab:
HCT(LAB) = [# of Red Blood Cells X Mean Cell Volume]/[Total Volume of the Sample]

Since the Red Blood Cell Count divided by the Total Volume of the Samples in each case are a constant, then we can divide the CLM equation by the lab equation. This results in:

HCT(CLM)/HCT(LAB) = 91/MCV

Put another way:
HCT(CLM) = [HCT(LAB) X 91]/MCV

or

HCT(LAB) = [HCT(CLM) X MCV]/91

When corrected for MCV, the CLM and LAB should agree within the standard deviations of the devices.

lbarrett, thanks for the additional info it will be most helpful in the clinic

Dear Lee,
Thanks very much for your useful reply. I appreciate your help
Nancy

Hello Crit-line users. Here is a tid-bit of information that may be of clinical worth. Red Blood cells seem to change shape or crenate with the addition of different preservatives. Heparin has demonstrated little to no effect but is not a long term preservative like EDTA or CPD. When whole blood or Heparinized blood is compared to a sample immediately treated (within, say 10 to 15 minutes) with EDTA (purple topped tube), the EDTA HCT has averaged about a -.7 HCT shift below the original blood. This effect of EDTA is virtually immediate and does not change much more, if any, with time.

The effect of CPD is almost double that of EDTA. CPD treated blood measured about -1.5 HCT below the original whole or Heparinized blood.

To avoid confusion, the Crit-line monitor is calibrated, on the average, to reflect the HCT of EDTA treated blood (purple topped tube) which is commonly used for laboratory draws.

Recently a clinic asked if there is an upper MCV limit, above which the correction formula(s) for Crit-Line HCT will not yield the correct answer. The answer is yes. The correction formula(s) have been shown to be correct up through an MCV of 104.

The higher the MCV value, the higher the probability of red blood cells bursting - causing free hemoglobin to be present in the blood plasma. This plasma free hemoglobin exhibits the same optical absorption properties in the blood as if it was still contained within the red cells. Since the Crit-Line uses optical absorption (and scattering) to measure HCT, it is not possible for the Crit-Line to fully distinguish between free hemoglobin in the blood versus that contained in the red blood cells. Therefore, if a patient has an extremely high MCV (the clinic that called has one at at 127!) then there is probably plasma free hemoglobin within the blood stream that the Crit-Line will measure as though it was still contained in the red cells.

When this patient's blood is measured by a cell counter, the hemoglobin that is not contained within the cells is disgarded and ignored. Therefore, the cell counter will find sustantially less red cells than indicated by the hemoglobin based, absorption method used in the Crit-Line. As a result, the Crit-Line will read HCTs substantially higher than the cell counter determines.

The Crit-Line correction formula(s) can be used to around an MCV of 110 with progressively increasing error if the patient's red cells are bursting.

Lee,
Thank you very much for your insightful respone. Your comments are very much appreciated.
Nancy