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Stepwise Success

Calculations

Published September 8, 2009 6:17 AM by Scott Warner

Most of us aren't mathematicians. Well, I'm not. I've known people who are just better at "seeing" math than myself. I've always struggled to add a column of figures in my head, forget solving Fermat's Last Theorem.

Which has been solved already. Just as well.

Even so, some formulas are burned into my layman's brain from repetition. Creatinine clearance, expressed as mL/min cleared and often corrected for body surface area, compares the level of creatinine in the urine with the level in the blood to estimate the filtration rate of the kidneys. Many years ago I memorized:

U*V
P

Where U is the urine creatinine, V is the volume divided by the number of minutes in the collection period (usually a day or 1440), and P is the plasma (serum) creatinine. "UV over P" was doubtless memorized by most if not all techs of my generation.

I wonder if this is still true. These days, chemistry analyzers and information systems perform this standby calculation, including a variety of ways to estimate the patient's body surface area. Easy access to this technology – even on handheld devices – makes "UV over P" less essential, even problematic.

Creatinine clearance depends on an accurate 24-hour (or other timed) collection, something which the laboratory has no way of verifying. Like so many laboratory tests, the preanalytical variables can be significant, and patient compliance is a big one. Your laboratory may have decided to report an estimated GFR calculation based on the serum creatinine as suggested by the National Kidney Disease Education Program.

If you look at these equations, they are not "UV over P" and are unlikely to be memorized. Indeed, they are impossible to report routinely in your laboratory without the help of an information system. And as I wrote in ADVANCE, it can be done.

Most of us aren't programmers, either. I suppose I should be happy that computers are memorizing these new equations instead of me.

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About this Blog


    Scott Warner, MLT(ASCP)
    Occupation: Laboratory Manager
    Setting: Critical Access Hospital
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