The determination and interpretation of reference intervals for multichannel serum chemistry tests

Journal of Family Practice, March, 1998 by James W. Mold, Cheryl B. Aspy, Kenneth E. Blick, Frank H. Lawler

BACKGROUND. When interpreting the results of clinical chemistry tests, physicians rely heavily on the reference intervals provided by the laboratory. It is assumed that these reference intervals are calculated from the results of tests done on healthy individuals, and, except when noted, apply to people of both genders and any age, race, or body build. While analyzing data from a large screening project, we had reason to question these assumptions.

METHODS. The results of 20 serum chemistry tests performed on 8818 members of a state health insurance plan were analyzed. Subgroups were defined according to age, race, sex, and body mass index. A very healthy subgroup (n=270) was also defined using a written questionnaire and the Duke Health Profile. Reference intervals for the results of each test calculated from the entire group and each subgroup were compared with those recommended by the laboratory that performed the tests and with each other. Telephone calls were made to four different clinical laboratories to determine how reference intervals are set, and standard recommendations and the relevant literature were reviewed.

RESULTS. The results from our study population differed significantly from laboratory recommendations on 29 of the 39 reference limits examined, at least seven of which appeared to be clinically important. In the subpopulation comparisons, "healthy" compared with everyone else, old ([is greater than or equal to] 75 years) compared with young, high ([is greater than or equal to] 27.1) compared with low body mass index (BMI), and white compared with nonwhite, 2, 11, 10, and 0 limits differed, respectively. None of the contacted laboratories were following published recommendations for setting reference intervals for clinical chemistries. The methods used by the laboratories included acceptance of the intervals recommended by manufacturers of test equipment, analyses of all test results from the laboratory over time, and testing of employee volunteers.

CONCLUSIONS. Physicians should recognize when interpreting serum chemistry test results that the reference intervals provided may not have been determined properly. Clinical laboratories should more closely follow standard guidelines when setting reference intervals and provide more information to physicians regarding the population used to set them. Efforts should be made to provide appropriate intervals for patients of different body mass index and age.

KEY WORDS. Reference values; clinical chemistry; laboratory; data analysis. (J Fam Pract 1998; 46:233-241)

Serum chemistry panels, made possible by multichannel clinical chemistry analyzers, have revolutionized the manner in which physicians evaluate patients. After complete blood counts and urinalyses, they are the most frequently ordered laboratory tests in both primary care and hospital settings. It is generally assumed that the laboratory's reference intervals for the tests included in such panels contain the central 95% of the values obtained from a healthy population. Based on this assumption, it has been calculated that one can anticipate, for a given healthy patient, a probability of at least one abnormal result, given n separate tests in the panel of 1-[(0.95).sup.n]) (assuming that the results of each test are independent of the others, which, of course, they are not).[1] It is also assumed that the reference intervals, with a few exceptions (eg, uric acid and gamma-glutamyl transpeptidase) apply equally well to men and women, whites and African Americans, young and old adults, and that they are unaffected by body mass.

During analysis of the data from a study designed to measure the effects of a screening panel (Chem 25) offered as a promotion by a local insurance company, we had the opportunity to test these assumptions regarding the reference intervals for the tests included in the panel. We reviewed distributions of test results in our population and compared them with the laboratory's reference intervals for 20 of the serum chemistry tests. In addition, we contacted four other laboratories to determine the procedures used by them to set reference intervals.

METHODS

Between February 1 and March 15, 1994, the Oklahoma State and Educational Employees Insurance Program offered its approximately 100,000 adult members older than 25 years the opportunity to have a Chem 25 test and lipid profile for $15. Phlebotomy sites were established in 300 locations throughout the state, and 8818 enrollees chose to have the blood tests done. Participants were asked to fast for 8 to 12 hours before testing. The tests were performed by a single laboratory in Oklahoma City using a DAX multichannel analyzer (Miles Inc, Tallytown, NY). Procedures for the storage and transfer of specimens were standardized and in accordance with the procedures of the laboratory that performed the analyses.

Prior to having blood drawn, each individual was invited to participate in a research project involving the completion of questionnaires before and again several months after the blood chemistry testing; 4150 subjects agreed to participate and completed the first questionnaire at the time that their blood was drawn. Follow-up questionnaires were mailed on July 22, 1994. The research participants who had not returned their follow-up questionnaire by October 4, 1994, were sent a second questionnaire. Response to the first mailing was 1746; an additional 576 responded to the second mailing for a total of 2322 persons with both initial and follow-up questionnaire data (response rate = 56%). Late responders were somewhat older than early responders, but they were similar in respect to sex, prior history of heart attack, family history of heart attack, systolic blood pressure, diastolic blood pressure, height, and body mass index. It seems reasonable to assume that responders were younger and healthier than nonresponders.