According to the bible of clinical pathology, Henry’s Clinical Diagnosis and Management by Laboratory Methods, around 70 percent of medical decisions are based on laboratory results. But how reliable are they?
By Thaddeus C. Hinunangan, M.D.
Certain variables and errors done during the collection, handling, transport, processing, and even reporting of laboratory tests may yield erroneous and misleading results that may put the patient at risk.
There are three major stages wherein errors can occur: the pre-analytic, analytic, and post-analytic phase. Of the three, the preanalytic phase is most affected by variables which range from patient conditions to technique of extraction. Several studies have identified that 32 percent-72 percent of all testing errors occur in the pre-analytic phase. (Lapworth,et. al., 1994; Stahl, et. al., 1998; Hofgartner, et.al., 1999; Bonini et. al., 2002; Plebani, 2010)
This stage refers to the time before the sample is extracted (including special preparations for certain tests) to collection and storage. It precedes the actual analytic stage wherein the sample is processed manually or by machine. The pre-analytic stage is affected by patient variables such as physical activity, diet, age, circadian variations (for example, hormones like cortisol are lowest in the evening and peaks early morning), medications, and even smoking.
Among the common sources of error is inadequate preparation. For example, lipid analysis requires at least 12 hours fasting. Chylomicrons are present postprandially up to six to eight hours, rendering the Triglycerides elevated. Another error which may occur even before collection is misidentification of the patient. Proper patient identifiers such as arm bands must be present and confirmed prior to extraction of the specimen. The specimen container must also be properly labeled prior to collection to prevent switching with samples of other patients. Time and date are also noted since certain substances tested may degrade over time.
During collection, common errors include wrong container especially for blood samples, wrong ratio of the specimen sample and additive, and wrong order of draw and improper technique.
Decoding blood tests
Test tubes for blood samples are color-coded according to the anticoagulant or additive used. Red tops are plain tubes for serum chemistry/ serology. Lavender tops have ethylenediaminetetraacetic acid (EDTA) additive which chelates calcium; this is the tube used for hematology tests. Blue tops have sodium citrate which also binds to calcium, and they are usually used for coagulation tests. Green tops contain heparin as an anticoagulant which inhibits thrombin formation, used for chemistry studies. There are still other less commonly used tubes, but the above mentioned are the most common. The specified ratio of specimen to additive must also be observed by making sure the specimen added must not exceed the indicated sample level, a line at the side of the tube.
The proper order of draw must also be observed for multiple blood tests ordered for a single patient. First are always blood-culture tubes to minimize contamination, next are coagulation sodium citrate tubes which must immediately be mixed otherwise coagulation tests might be rendered inaccurate. Heparin tubes are next, and lavender tubes are last. However, if there are gray tops (glycolic inhibitor tubes) to be used, they can be the last in order.
To safeguard the accuracy of test results, strict quality measures must be implemented and certain specimens must be rejected. Reasons for rejection may include: hemolyzed specimens in which intracellular contents are released and thereby raising levels of measured electrolytes like potassium. Clots which are present in anticoagulated specimen because these may be aspirated by the machine and clog the tubes. A non-fasting specimen when fasting is required should be disallowed and the patient should have blood redrawn only when specific preparations have been made. Use of improper tube, wrong volume, or improper transport conditions may also be basis for rejection. Unlabeled or mislabeled specimens must also be rejected. Lastly, contaminated or leaking containers must be returned.
Errors in analytic phase are largely avoided by machine calibration and quality control, and such topics are beyond the scope of this article. Post-analytic errors may involve the laboratory information system which now delivers the results to the clinicians.
Laboratory testing is now ubiquitous. The proper techniques and utilization will allow the most accurate information to reach clinicians and aid in the patient diagnosis and management.
“Several studies have identified that 32 percent-72 percent of all testing errors occur in the pre-analytic phase”
June 2018 Health and Lifestyle