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Who/what is NORMAL?
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There is a must-read article in the current issue of the journal Clinical Chemistry, the journal of the American Association for Clinical Chemistry. The authors question both the origin and validity of cut points in the clinical chemistry lab. I won’t try here to summarize their editorial but simply encourage you to get a copy, read it and learn about PSA cut points.
What exactly is the problem? If the reference interval for potassium in your institution’s lab is 3.5 mEq/L to 5.4 mEq/L and the patient’s value is 3.4 mEq/L or 5.5 mEq/L, it is highlighted as “abnormal” when the results are posted. Increasingly and appropriately the patient gets a copy of the lab report sent to their home and they can’t miss these abnormal labs.
How are these reference intervals established? A sample is obtained from a number of “healthy” persons, free of any disease and their lab value for a particular analyte (eg, potassium) or group of analytes (eg, electrolyte panel) is measured. The sample size is usually determined by simple practicality — how many volunteers can you find and how much does it cost to run the assays? The next step is to determine whether the values are normally distributed around a mean value — a bell shaped curve — and the 95% confidence interval established. That simply means that 2.5% of the group of healthy volunteers will have a value below the lower limit of “normal” and 2.5% will have a value above the upper limit. Remember also that a sodium of 134 mEq/L is not 134 ± 0.
Just think of the total number of electrolyte profiles performed in your institution’s clinical chemistry lab each year and understand that a minimum of 5% will have their “abnormal” value flagged — for each of the four electrolytes. We all know how to handle that and can put that into a clinical perspective when the patient sees his or her number is “abnormal." But how often do we feel obligated to repeat the test?
Electrolyte values are not part of most patients' concerns. Now let’s try that same approach with cut points for a lipid profile or fasting blood glucose. Our patients know full well the implications of an abnormal lipid profile or fasting blood sugar. They also know at least one person with a bad outcome from coronary artery disease or diabetes, often a family member. Our skills as clinicians need to come to the forefront, and we need to put minimally abnormal numbers into the perspective of everything we know about the patient. Management should not be dependent on that lab number alone.
We should become smarter about obtaining “routine” labs, though of course our competence as clinicians is increasingly being judged on precisely that!
Dr. Kleerekoper has disclosed that he is a member of AACC and has served on the Editorial Board of Clinical Chemistry.