Reference Intervals vs. Reference Change Values

If we didn’t use reference intervals (RI), how would we know whether a person is “normal” or not? Or more accurately, how would we know whether a lab test result indicated health or disease? Reference intervals have been around as long as lab tests and they help clinicians diagnose and monitor a patient’s disease state. .

Most RI are developed using a specific patient population and should be used only with that population. However, some RIs are “health-based,” such as cholesterol and vitamin D. Both these analytes have RI that indicate what amount of the analyte should be present in a healthy individual, not how much is present in your specific population of patients. In general, health-based RI can be utilized in all populations, as long as the analyte assays are commutable. Thus these type of RI are often more useful than population-based intervals.

But should we be using reference intervals at all? One problem with population-based RI is that any given individual’s values may span a range that covers only part of the population RI due to biological variability. For example, an individual’s creatinine may be 0.6 – 0.9 mg/dL regularly. Since the RI for creatinine for his population is 0.4 – 1.4 mg/dL, a value of 1.2 mg/dL would not be flagged as be abnormal. However, 1.2 mg/dL may very well be an abnormal result for this individual We need to consider using reference change values (RCV) in addition to RI.

Reference change values are calculated values that are used to assess the significance of the difference between two measurements. Essentially, a RCV is the difference that must be exceeded between two sequential results for a change to be a significant change. The calculation requires knowledge of the imprecision of the analyte assay (CVA) and the biological variation (CVI) of the analyte. The formula for calculating RCV is: RCV=21/2 · Z · (CVA2 + CVI2)1/2 , where Z is the number of standard deviations for a given probability. Luckily, labs know the imprecision of their assays and there are tables available for biological variation.

It’s very likely that neither RI nor RCV by itself is adequate for interpreting analyte results. Using both may be a better alternative, especially using RCV for monitoring disease progression or therapeutic efficacy. Flagging sequential values that exceed the RCV—and reporting this change—should be considered.

-Patti Jones

Who Regulates Laboratory Developed Tests?

A laboratory developed test (LDT) is any test that has been developed by an individual laboratory, often using instruments and/or reagents that have not been approved by the FDA for use as/in an in vitro diagnostic test. For example, measuring pH using a pH meter and pH calibrators from a scientific supply company is an LDT. So is performing a spun hematocrit,  measuring acylcarnitines by tandem mass spectrometry, or performing newborn screening on dried blood spots. Even using an FDA-approved assay for samples or in a manner not specified by the manufacturer makes that assay an LDT. If you look around your lab, you may find that you’re performing an LDT without really thinking about it.

Who regulates these tests? The FDA regulates in vitro diagnostic testing, and LDTs fall under their purview. Until recently the FDA has used “enforcement discretion” and has essentially allowed CLIA regulations and CLIA oversight to ensure proper validation and monitoring of LDTs. CLIA regulation Subpart K, Section 493.1253 gives the specific parameters that must be properly validated in any non-FDA-approved assay. CLIA also regulates the proper usage and control of LDTs, just like any test performed in the laboratory. Is it necessary for LDTs to be regulated more highly than this?

In June of 2010 the FDA announced its intention of taking a more active role in LDT regulation in the future. They also held a public meeting to discuss their increased oversight. All laboratories which perform LDTs will do well to monitor developments in this newly intended enforcement of the FDA’s role, and keep abreast of changes coming out in the regulatory environment for these tests.

-Patti Jones