Mchc

39%

Platelet

340,000 X 109/L

While the other results were pending, the CBC was run on automated instrumentation. The operator noticed that the hemoglobin and hematocrit failed the correlation check by not adhering to the rule of three: hemoglobin X 3 = hematocrit ±3%. Because the hemoglobin was significantly elevated in comparison to the hematocrit, the hemoglobin value was suspect. The patient sample was spun and observed for lipemia. The plasma was cloudy and grossly lipemic, and when remixed, a milky appearance could be seen in the mixed sample. Lipemia interferes with the optical measurement of hemoglobin, giving a false elevation of hemoglobin, and all subsequent measurements that depend on a hemoglobin value in the calculation, namely MCH and MCHC. It became clear that corrective action needed to be taken to provide valid results. Corrections for lipemia can occur via one of two methods: the plasma blank correction method or the plasma by dilution replacement method. The most frequently encountered method is the plasma blank correction method, in which the sample is spun, a plasma aliquot is removed, and then the spun plasma is recycled through the instrument. This plasma value is used in the following formula to correct the hemoglobin result

Corrected hemoglobin = Initial whole blood hemoglobin-(Plasma hemoglobin blank X [1 — Initial whole blood hematocrit/100])

When our plasma blank was run, we obtained a value of 3.0.

Therefore, our correction would be as follows: = 15.5 - (3.0 X [1 — 0.39]) = 15.5 — 1.83 = 13.7

This hemoglobin value is now used in the calculation for MCH and MCHC, to yield corrected MCH and MCHC results of 28.3 and 35.1, respectively

The second corrective method is the diluent replacement method. Once the whole blood has been cycled through the automated instrument, an aliquot of the sample is removed and spun. The plasma from the

(Refer to normal values in Chapter 2.)

spun sample is carefully removed and replaced by an equal amount of saline or other diluent. The removal of plasma and replacement with saline are critical in this procedure. If this step has been done accurately, there will be little difference in the red cell count and this will serve as a quality control for the accuracy of pipetting in this method. Too wide a variation in the red cell counts will indicate poor pipetting. Once an accurate replacement has been established, the saline sample is cycled and the hemoglobin can be reported directly from this sample and used to recalculate indices.

Either of these methods involves labor-intensive, yet essential, steps in reporting an accurate CBC. The operator must first recognize the discrepancy between hemoglobin and hematocrit and then be familiar with the corrective steps that need to be taken to provide a reliable CBC.11

(Refer to normal values in Chapter 2.)

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