[Analytical uncertainty--how wrong can a laboratory result be?]. / Analytisk usikkerhet--hvor stor feil kan laboratoriesvaret ha?

Some uncertainty encumbers the outcome of all laboratory tests. Patient conditions and handling of specimens, as well as analytical variation and systematic error will affect the results. In order not to confuse the interpretation of test results, the analytical standard deviation should not exceed one half of the intraindividual biological standard deviation, and systematic error should not exceed 1/16 of the reference interval. However, these goals cannot always be achieved. Moreover, analytical control procedures have limited ability to detect errors in the analytical process. After one analysis of control material, using +/- 2 standard deviations (analytical) from the mean as acceptance limits, the magnitude of a systematic error must be 3.3 times the analytical standard deviation in order to be detected with 90% probability. As a result of such error, patients' results can be released with a total error up to five times the analytical standard deviation. More complicated control procedures may give smaller total error. Practising physicians should be familiar with the variations in lab results, and interpret the results accordingly. Lab results that conflict with results of other investigations should be used with caution.

Within-subject biological variation of reticulocytes and reticulocyte-derived parameters.

Automation of reticulocyte counting has decreased the analytical imprecision of this parameter. This has made it possible to use the number of reticulocytes and its derived parameters (reticulocyte maturity and reticulocyte cell indices) in new diagnostic and monitoring situations. For rational use of these parameters, it is important to have knowledge of their biological variability. The biological variability of reticulocytes and its derived parameters was studied in 13 healthy people during a period of 7 wk on 2 different instruments. The within-subject coefficient of variation for the reticulocyte count was about 11%, for the mean reticulocyte volume, mean reticulocyte haemoglobin content and mean reticulocyte haemoglobin concentration it was between 1 and 2%, whereas the coefficient of variations for the subpopulations of reticulocytes with different maturity varied depending on the method used for the measurements. The critical difference, that is the change in a result making it significantly different from the previous result, was about 35% for the reticulocyte count and 5-8% for the reticulocyte cell indices, making these indices excellent to follow changes in erythropoiesis. With a possible exception for the mean reticulocyte volume, the within-subject variation was small compared to the between-subject variation.

A comparison of the differential white blood cell count using the Technicon H1, the Technicon H6000 and the Coulter VCS.

One hundred blood samples were analysed on Technicon H1, Technicon H6000, and Coulter VCS hematology analysers, and the white cell differential counts were compared. All three instruments showed very good agreement for neutrophils and lymphocytes, good agreement for eosinophiles, and acceptable agreement for monocytes. For the material analysed there were no disparities of clinical significance between the results obtained from the three instruments.