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What is "normal"?

Posted Aug 26 2008 4:17pm

When it comes to laboratory values and medical testing, a common dilemma is knowing what is "normal." Let me explain.



First of all, when you receive a laboratory result for a test, a "reference range" or "normal range" is usually provided. Where did that range come from?



It varies from test to test. For instance, a low potassium is easy, because low potassium levels can lead to life threatening consequences, e.g., dangerous heart rhythms. High potassium likewise, because dangerous phenomena develop when potassium generally exceeds 5.5 mg/dl or so.



But what about something like HDL or LDL. Here's where confusion reigns. Often, "normal" is obtained by taking the average and saying that any value plus or minus two standard deviations (remember that painful class?) represents normal or reference range.



If that were true, what if we applied that principle to body weight. If we weighed several thousand adult women, the average would be in the neighborhood of 172 lbs (no kidding). Does that mean that 172 lbs plus or minus two standard deviations is normal? No, of course not.



There is therefore a distinction between "normal" and "desirable". For HDL cholesterol, your laboratory report might say that an HDL cholesterol of 40-60 mg/dl is normal. But is it desirable? I don't think so. The most frequent HDL level for a male with a heart attack is 42 mg/dl--hardly desirable.



Let's take triglycerides. The average triglyceride level in the U.S. is somewhere around 140 mg/dl. For those of us who do a lot of lipoprotein testing, we can tell you that triglycerides at this level, though generally regarded as being within the normal range, are associated with flagrant and obvious excesses of several abnormal lipoprotein particles that contribute to coronary plaque growth (VLDL and often IDL; small LDL; drop in HDL and shift towards small HDL).



So, always take the so-called "normal" or "reference" values on a lab report as crude guidelines that often have little or nothing to do with health or desirability. Unfortunately, many physicians are not aware of this and will declare any value within the normal or reference range as okay. An HDL of 40 mg is not okay. A triglyceride level of 140 mg is also not okay.



What is okay? What is desirable? That depends on the parameter being examined. From a basic lipid standpoint, of course, we regard desirable as 60-60-60. Desirability from a lipoprotein standpoint we will cover in a more thorough Track Your Plaque Special Report in future.

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