Reference ranges provide an expected range of values in the general healthy population for a given analyte. (See the image, where the reference ranges are on the right side of the lab report.) Laboratories must verify each test method's reference ranges for their target population. (For example, a pediatric hospital would not report adult reference ranges as suggested by the method manufacturer.)
- Reference ranges (normal values) should reflect the mean value in the population and a certain level of variation (usually 2 standard deviations).
- 95% of all normal patients will fall within the reference range of an analyte. Values outside the reference ranges may indicate an abnormality in the patient or a problem with a given set of test results.
(More on standard deviation is presented in the next topic section.)
The Importance of Monitoring Patient Results: XbarM and Reference Ranges
In addition to monitoring internal quality control results, patient results should be monitored.
- As a general example: Should 20% of patient results suddenly begin to exceed a given reference range, there is most likely a testing error.
X-barM is an example of a control tool (computer program algorithm) that continuously monitors analyzer performance and is capable of detecting any changes in real-time.
- The X-barM process is also referred to as a system of moving averages (when referring to patient results averaged over time).
- Laboratory analyzers that allow for X-barM QC calculations should contact the manufacturer's technical specialist for help in setting data collection targets.
To summarize, systemic errors can be detected when patient results are monitored over time.
