Haematology Watch


Dr. Mehmood ul Hasan Malik


Accuracy. “Closeness of the agreement between the result of a measurement and a true value of the measurand.

Usually expressed in the same units as the result, as the difference between the true value and the value, or as a percentage of the true value that the difference represents; expressed this way the quantity is more correctly termed ‘inaccuracy.’” [CLSI] This is a total error definition of accuracy that encompasses both the random and systematic errors in a measurement, in contrast to the more traditional (older) concept that accuracy was related only to systematic error (and is now being called “trueness”). In the context of the method validation here, the terms random error, systematic error, and total error are generally used to clarify the intent and meaning.

Bias. A systematic difference or systematic error between an observed value and some measure of the truth.

Generally used to describe the inaccuracy of a method relative to a comparative method in a method comparison experiment. It also has a specific meaning in the statistical t-test, where bias equals the difference between the mean values of the two methods being compared or the average of all the differences between the paired sample values.

Calibration.“The process of testing and adjustment of an instrument, kit, or test system, to provide a known relationship between the measurement response and the value of the substance being measured by the test procedure.” [CLSI]

Carryover. The percentage of sample remaining in the system and picked up by the next sample cycled.

Control. Material of known approximate value used to assess the stability of system.

Delta limit. Measure of acceptable difference in a patient's recent and previous test result based on biological variation and instrument's inherent imprecision over a defined time.

External Quality Assessment. A method that allows for comparison of a laboratory’s testing to a source outside the laboratory.

Error. The difference between a single estimate of a quantity and its true value.

Imprecision. The standard deviation or coefficient of variation of the results in a set of replicate measurements.

Inaccuracy. The systematic error estimated by the difference between the mean of a set of data and the true value.

Internal Quality Control. An analysis programme, using quality control samples, that is used to verify acceptability and stability of laboratory results.                                                                           

Levey-Jennings control chart. visual tool for evaluating quality control data in the context of previous results where data are plotted relative to the ± 2 SD on the vertical axis versus days on horizontal axis.  commonly used control chart in which individual control measurements are plotted directly on a control chart with limit lines drawn either as mean ± 2s or mean ± 3s. Time is displayed on the x-axis usually in terms of days or runs.

Precision. The agreement among replicate measurements.

Proficiency testing: A program in which an external agency provides unknown samples for analysis, and the results are returned to the participant with an evaluation of Acceptable or Not acceptable performance.

Quality Control. Monitoring specific project results for their compliance with relevant quality standards while eliminating causes of unsatisfactory results.

Random error, RE. An error, either positive or negative, the direction and exact magnitude of which cannot be exactly predicted.

Reference material, RM“A material or substance, one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials.” [CLSI] See also certified reference material.

Reference method“A thoroughly investigated method, in which exact and clear descriptions of the necessary conditions and procedures are given for the accuracy determined of one or more property values, and in which documented accuracy and precision of the method are commensurate with the method’s use for assessing the accuracy of other methods for measuring the same property values, or for assigning reference method values to reference materials.” [CLSI]

Sample. "The appropriate representative part of a specimen which is used in the analysis." [IFCC]

Shift. An abrupt and sustained change in one direction in control values.

Specimen. "Material available for analysis." [IFCC]

Standard. "Material or solution with which the sample is compared in order to determine the concentration or other quantity. The compound term calibration standard should be used whenever needed to avoid confusion with other technical or colloquial meanings of the word standard." [IFCC]

Standard deviation. Square root of the variance.

Systematic error, SE. An error that is always in one direction and is predictable.

Total error, TE. The net or combined effect of random and systematic errors.

Trend/Drift. A gradual change in the test results obtained from control material.

Westgard rules, Westgard multi-rule control procedure. A control procedure that uses a series of control rules to test the control measurements. A 1:2s rule is used as a warning, followed by use of 1:3s, 2:2s, R:4s, 4:1s, and 10x as rejection rules.

to be continued