Friday, January 18, 2013

Quality control in Biochemistry laboratory

Quality Assurance in the biochemistry laboratory is intended to ensure the reliability of the laboratory tests. The objective of quality assurance is to achieve reliable test results by 

  • Accuracy
  • Precision

This refers to the closeness of the estimated value to that considered to be true. Accuracy can, as a rule, be checked only by the use of reference materials which have been assayed by reference methods.

This refers to the responsibility of the result, but a test can be precise without being accurate. Precision can be controlled by replicate tests and by repeated tests on previously measured specimens. And the test result or value which we get should be closer to the previous one.

Inaccuracy and/or imprecision occur as a result of using unreliable standards or reagents ,incorrect instrument calibration, or poor consistently faulty dilution or the use of a method that gives a reaction that is incomplete or not specific for the test.

First of all, Edward Demming gave the idea about quality control. According to him,

Improved quality=increased productivity at lower cost.

This can be done by
  1. Eliminating rework
  2. Save time
  3. Save labour
  4. Save material e.g. reagent, specimen etc.
  5. Patient care.
Quality Assurance Programme Includes
  • Internal quality control (IQC)
  • External quality assessment (EQC)
  • Proficiency surveillance
  • Standardization
Internal quality control

This is based on monitoring the Biochemistry test procedure that is performed in the laboratory. It includes measurement on specially prepared materials and repeated measurements on routine specimens as well as statistical analysis day by day of date obtained from the test which has been routinely carried out. There is thus continuous evaluation of the reliability of the work of the laboratory. Hence IQC primarily check the precision of lab work.

External quality assessment
This is the evaluation by an outside agency of the performance by a number of laboratories on specially supplied samples. Analysis of performance is retrospective. The objective is to achieve between lab and between method compatibility, but this doesn’t guarantee accuracy unless the specimens have been assayed by a reference lab alongside a reference preparation of known value. Schemes are usually organized on a national or regional basis. Hence, EQA is mainly concerned with analytical part of the test.

Proficiency surveillance

This is concerned with various aspect of laboratory apart from analysis part i.e. this ensures adequate control of the pre and post analytical stages of test. It implies critical supervision of all the aspects of laboratory tests. Such as,
  • Sample collection
  • Labeling
  • Delivering
  • Storage
  • Reading
  • Reporting
  • Establishment of normal reference values.
  • Maintenance and control of apparatus and instruments etc.
This refers to both materials and reference methods.
A material standard or reference preparation is used to calibrate analytic instruments and to assign a quantitative value to calibrators.

A reference method is an exactly defined technique which provides sufficiently accurate and precise data for it to be used to assess the validity of other methods. The main international authority concerned with material standards are WHO and international council for standardization in hematology (ICSH). The material prepared by these authorities are international standards(international reference preparation)and are of primary standard. These international standards are not freely available and are not intended for routine use, but this serve as standards for assigning values to commercial standard which is of secondary standard.

Something that can go wrong
  • In writing the order
  • Transcribing the order to lab.
  • The requisition from floor to the lab.
  • Collecting the sample (IQC, PS, and Std.)
  • Handling after collection(PS)
  • Running the test (predominantly to IQC).
  • Sending the data back to the floor.
  • Putting the data on the chart.
Awareness that has to be adopted for quality control can be divided into 2 groups.

1. Primary Goals
  • To report out all correct data.
  • Not to report the incorrect patient values.
2. Error Detection
  • To detect error before it leaves the lab.
Errors in quantitative system
There are two types of errors in quantitative system
  1. Random error
  2. Systematic error.
Random Error. (Inherit Error, Noise Error, Background Error)

This is the error in which there will be variation in test result /data on either side of mean. In other words the values obtained will be low as well as high to the mean value or true value.

This may be due to
  • Slight variation in line voltage, lamp output or temperature.
  • Slight variation in pipettors and dispensers.
This error is measured by standard deviation (SD) and coefficient variation (CV).

Systemic Error

This is the error in which variation occurs in one direction away from the true value i.e. either value goes up or down. The difference between measured value and true value is called Bias
Systemic errors are errors within the test system or methodology.
  • Assigned value to calibrators.
  • Reagent composition
  • Antibody specificity.
Six components of error detection system
    • Patient identification, sample collection and handling.
    • Analytical method.
    • Instrument maintenance
    • Control material.-In build error detecting test, it is run as per the test and its value is known so helps to detect whether our test is correct or not.
    • Quality control monitoring.
    • Clerical.
Quality control material
  • It is a known sample whose range of values has been established prior to the test either by international authorities or by commercial firms.
  • This control sample is inserted into the testing process, being exposed to the same condition as the patient sample and value is measured. If the values of control material is with in the range then it is said that the test procedure of the error detectors.
Standard deviation
If values are assigned to a same specimen a number of times by repeating the test, the dispersion of results around the mean will indicate the error of reproducibility. And deviation of values from the mean is called standard deviation. This gives an idea about precision (random error).95% of result should be with in ±2 SD. 99.7% of results should be with in ±3 SD. To determine SD, 10-20 identical tests are carried out on the same sample.

Gaussian distribution
It is a bell shaped curve resulting from events or data which occur symmetrically about the mean when frequency and data are plotted.
SD= *
X…test value
X*...Mean value
N... number of test
Extent of spread of measurement about mean is SD.

Mean gives idea about the accuracy of result i.e. systematic error.

Shewart levey jenning chart
  • Runs or days are plotted on x axis.
  • Values obtained from analytic run are plotted on Y axis.

    • Samples of control specimens are included in every batch of patient’s specimen.
    • Mean and standard deviation are then established.

    Satisfactory results are obtained only when
    • Sequential results oscillate about the mean
    • Less than 5% of results will fall outside of ±2 SD 
  • If 2 or more results are on outside ±2 SD.
  • Consecutive values may get increased or decreased.
  • Consecutive values shouldn’t on one side of mean.
Cumulative sum method (CUSUM)
  • For this 20 identical tests are run.
  • Any slight error can be detected with CUSUM chart.
  • Any change in the calibration will be detected as an alteration in the slope of this line.
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