Clinical Laboratory - Fundamentals

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Preanalytical, Analytical and Postanalytical Variables

Preanalytical, Analytical, and Postanalytical Variables Affecting Error: Variability with Phlebotomy; and Patient Identification

Preanalytical variables occur prior to specimen testing and may include variables involving the process of obtaining a specimen.

Analytical variables occur during actual testing of the specimen. Performance of tests in the laboratory is rigorously controlled, with quality control procedures in place that markedly reduce errors in the analytic phase of testing. Differences in the methods for performing tests in different laboratories may yield slight differences in reported results or normal ranges, so check reference ranges given for the test.

Postanalytical variables occur after a test result is generated. In general, suchpostanalytical errors occur with entry, manipulation, and reporting of test data. If results are written by hand or entered via keyboard, then an entry error may occur. Errors in data transmission include relaying or hearing verbal information incorrectly. Reporting problems include exceeding the turnaround time for reporting or not notifying the appropriate health care provider.

Patient Identification

The most serious error is failure to properly identify the patient. Even if everything else is done perfectly, the final result will not apply to the patient incorrectly presumed to be the source. Additional clerical errors include missing the order and failure to obtain the test, or misinterpreting the order.

Two patient identifiers must be present. Name and identification number are the most common. Electronic identification technology, such as bar codes or RFID, will suffice if they record two or more person-specific identifiers.

Mis-identification of the patient may lead to inappropriate testing or inaccurate result reporting, so that a missed diagnosis or therapeutic misadventure with adverse outcome may occur.


Variability may be introduced during phlebotomy: using the wrong specimen tube (e.g., wrong type and concentration of anticoagulant for specimen collection) and the effect of drawing blood from an infusion site.

Hemolysis can occur if the tubes with anticoagulant additives are not mixed gently 5-10 times; if blood is drawn from a hematoma; frothing of the sample; if the venipuncture site is not dry; with probing of the site; or traumatic venipuncture. Indwelling lines or catheters are a potential source of test error because most lines are flushed with a solution of heparin to reduce the risk of thrombosis.

Hemoconcentration from an increased concentration of larger molecules and formed elements in the blood may be due to several factors: prolonged tourniquet application (no more than 2 minutes), massaging, squeezing, or probing a site, long-term IV therapy, or sclerosed or occluded veins.

Prolonged tourniquet application will cause hemoconcentration of non-filterable elements (i.e. proteins). The hydrostatic pressure causes some water and filterable elements to leave the extracellular space. Significant increases can be found in total protein, aspartate aminotransferase (AST), total lipids, cholesterol, iron. This also affects packed cell volume and other cellular elements in whole blood.

Failure to draw the sample into the proper tube can interfere with test results by subjecting the sample to improper coagulation or to chemicals that alter the compounds to be measured. Failure to draw samples in the proper order of tubes may contaminate samples.

  • Example: a specimen for a complete blood count should be drawn into a purple top tube, but drawing into a red top tube without anticoaculant will render the sample untestable.

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