“Normal Values”: How are Normal Reference Ranges Established? Module 5 “Normal Values”: How are Normal Reference Ranges Established?

Reference Ranges Reference Ranges are required by professional accreditation and regulatory standards Laboratory directors determine and evaluate reference or “normal” ranges reported with all test results.

Reference Ranges A reference range is used to interpret a lab test result. In most cases, a “normal range” is used as the test’s “reference range”. But sometimes, what is normal isn’t healthy… So a reference (but not normal) range is used e.g. total cholesterol desirable <200mg/ml

Establishing Reference Ranges When selecting the decision threshold or cutoff value (a limit above or below which a patient is considered affected by a particular disease, i.e., normal or abnormal), a variety of methods can be used.

Gaussian Distribution If tests results from a normal healthy patient population fall into a bell-shaped, Gaussian, normal distribution, the central 95% is usually used as the test’s normal range. For many (but not all) tests, this is how the range of tests results for normal healthy individuals is determined.

Some Normals are Abnormal …… and Vice Versa The normal range encompasses the mean plus or minus two standard deviations or, again, about 95% of normal, healthy individuals’ test results. However, 5% (roughly 1 out of 20) normal healthy patients may be outside the cutoff value. Roughly 2.5% of normal people can be expected to have a result below and roughly 2.5% of normal people can be expected to have a result above the reported normal range. This situation is encountered with almost all tests. This is because the distribution of tests results from normal, healthy individuals overlaps (to varying degrees) with the distribution of test results from sick patients with the relevant disease.

Two Populations of Results Theoretically, the better tests minimize this overlap between the distribution of normal and abnormal test results. An ideal test would have no overlap at all and could perfectly discriminate between a normal and abnormal test result. Lab experts continue to look for at least one test like this …….

Non-Gaussian Distributions For non-Gaussian distributions, lab directors can use other nonparametric techniques to establish reference range limits Example: set upper and lower limits of normal to include 95% of the population after all of the test results have been transformed into logarithms taking the central 95% of the transformed data.

Where’s the Threshold or Cut-off? Ultimately, where the lab places the limits (threshold or cut-off) on a normal or reference range determines what level of result is considered “normal” or “abnormal”.

In the next slide, the values for the concentration of a hypothetical analyte were determined in a group of 100 healthy persons and in a group of 25 diseased persons. The raw data for the group were fitted to gaussian distributions. A through D represent possible cutoff values that could be used to classify subjects based on the analyte values.

Distribution of a Test Result in Healthy (n=200) and Diseased (n=50) Persons Healthy Persons Frequency Cutoffs A B C D Diseased Persons 10 20 30 40 50 60 70 80 90 100 110 120 130 Analyte Value (units)

What would be the advantages of selecting A as the cut off? What about D?

Where’s the Threshold or Cut-off? To reiterate, where the lab places the limits (threshold or cut-off) on a normal or reference range determines what level of result is considered “normal” or “abnormal”. It also affects the distribution of values and how they are tallied in the predictive value grid shown on the next slide …..

Predictive Value Grid Test Result Disease or Sick No Disease (Normal, Healthy) Positive Result* True Positives False Positives Negative Result* False Negatives True Negatives *“positive” usually refers to a test being abnormal, “negative” usually refers to normal

The Next Module - Entitled Module 6 “Common Laboratory Tests” - is a reference for you as youbegin interpreting laboratory data in MHD.