Statistical quality control in hematology analyzers

The statistical quality control carried out in hematology analyzers has many important differences from the corresponding techniques in the clinical chemistry analyzers. These differences are due to reasons such as the high stability of cytometry technology, the small biological variation of some hematology parameters, the big reagent vials and the small time lasting of the hematology controls.

Because of the above reasons Levey-Jennings charts in hematology analyzers are different from corresponding charts in clinical chemistry. For instance the hematology Levey-Jennings charts have only three lines (upper and lower limits and central line). The reason is that these Levey-Jennings charts are not created statistically from a normal distribution of former quality control data, which is not possible because of the very small variation of hematology quality control values. In hematology analyzers the upper and lower control limits act as the “specification’s limits” in industry quality control.

The small biology variation of many hematology parameters made many researchers to established quality control methods based only on patients results. Such suitable parameters are the erythrocyte indexes (MCV, MCHC, MCV) with the smaller biological variation (due not only to biology but mostly to the hematology analyzer’s technology). These attributes of them inspired Brian Bull (an American Hematologist) to establish a new quality control method widely known as “Bull’s algorithm”.

Bull’s algorithm (also known asmethod) detects systematic errors in MCV, MCHC and MCV and consequently in HgB, Hct and RBC. His method is a kind of moving average. Its main idea is to estimate the mean value of the last twenty patients’ values, including in them the mean value of the batch of the previous twenty values. The algorithm itself is a quite complicated equation which eliminates the outliers and estimates the moving average of the last twenty values. Bull’s algorithm has been proved quite effective in detecting small systematic errors (almost 1%) not only in erythrocyte indexes but also in almost all the hematology parameters. It uses all patients’ data without exception. The last fact made Bull’s algorithm the cheapest quality control method in laboratory medicine.

Hematology quality control samples last only 20 – 30 days and are very expensive, when, on the other hand, whole blood samples are stable in the refrigerator for 24 hours. These facts led some researchers to find methods which are based on the repetitive analysis of patient samples. These methods are known as “retained patient specimens”.

In 1988 Cembrowski (Canadian clinical chemist) established the most effective “retained patient specimens” method. It was based on the repetitive analysis of the same patient samples between two successive days. His method is known as “m/n_lim”.

-       “Lim” stands for the quality control limit. It is equal to the double of the standard deviation of the repetitive analysis (2 x SD).

-       “n” stands for the number of patients’ samples which will be analyzed twice.

-       “m” stands for the portion of “n” number of samples which is permitted to be out of limits (“lim”).

Statistical simulations created by Cembrowski proved the effectiveness of his method. According to him the best combination of “m”, “n” and “lim” is 2, 3, 2 or 2/3_2s.

Concluding, three different methods are in the disposal of the laboratory in order to detect the analytical errors in hematology laboratory. Levey-Jennings detects systematic and random errors. On the contrary, Bull’s algorithm and “retained patient specimens” detect only systematic errors, but they have the advantage of the low cost. Laboratory can choose the best combination of the three.

 Tελευταία ενημέρωση:  Κυριακή Ιανουαρίου 20, 2013