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August 20th, 2006:

A Rational Scheme for Medical Laboratory Results

Medical laboratory results these days are a hodgepodge of numbers on various scales and with various units. For example, the Merck Manual lists various laboratory test normal ranges and their units:

Hematocrit: Male 41-50%, Female 35-46% Hemoglobin: Male 13.8-17.2 g/dL, Female 12.0-15.6 g/dL ... Sodium: 135-146 mmol/L  

These “normal ranges” can be sort of misleading. If your value is numerically half of the lower-end of the hematocrit range, for example (say, 20%), you would be sick but still alive. However, if you have only half the normal range of sodium concentration (say, 70 mmol/L), you’d be dead.

This is crappy. It imposes a high cognitive load on doctors by requiring them to know a variety of “normal” ranges, it makes lab results opaque to patients and the uninitiated, and it has a “hidden memorization cost” of knowing the implications of going outside the normal range (such as the difference between having half the normal measurement for hematocrit vs. sodium, above).

I propose a replacement scheme for all scalar laboratory values (at least those in the main test batteries, like the chem-N and CBCs). In my scheme, all “unit” lab results are replaced (realistically, augmented) by “rational” values. Rational values are normalized at 100 for the center of the range. The “normal range” is represented by the range 90-110. The values associated with roughly 50% mortality are set at 50-150. The ranges 80-120, 70-130, and 60-140 will be pegged at some statistics-based measurement, either based upon standard deviation or upon increased chances of negative outcomes, whichever an appropriate standards body decides best (there are some labs for which it might not make sense to have it be standard deviation-based, others for which it would).

The correspondence of “unit” to “rational” measurements is not necessarily linear; the formulae to determine this will be decided per-test, reviewed annually by the standards body, and published as an appendix to standard references and on the Web.

The “core rational” lab values are those which are unadjusted for average adults. “Adjusted rational” lab values are adjusted for sex and body mass. “Peds adjusted rational” values are adjusted as above but with age ranges.

All lab reports will show these values on the summary page; “unit” measurements will be provided as well (they will doubtless remain indispensable for certain purposes). Color-coding would be straightforward: green for +/- 10, yellow for +– 20, orange for +– 30, and red for +/- 40.

This will become an ever more crucial part of diagnosis as we move toward greater automation (e.g. field lab-testing machines that paramedics could carry) and de-skilling of the medical profession (nurse practitioners, paramedics, self-administered care and monitoring, etc.). It also becomes a key part of the understanding required for personal medical choice as we move the economics of health care toward a (partial) patient-pays model.

If someone wants to give me a grant for a year of my time with a couple of assistants, I’ll go ahead and set this up. Drop me an email – rlucas@tercent.com.