e2-03: Diagnostic Test Preparation

Factors affecting both the patient and the specimen are important. The most crucial element in a properly conducted laboratory test is an appropriate specimen.

Preparation of the patient is important for certain tests—eg, a fasting state is needed for optimal glucose and triglyceride measurements; posture and sodium intake should be strictly controlled when measuring renin and aldosterone levels; and strenuous exercise should be avoided before taking samples for creatine kinase determinations, since vigorous muscle activity can lead to falsely abnormal results.

Careful attention must be paid to patient identification and specimen labeling—eg, two patient identifiers (name and birth date, or name and unique institutional identifier) must be used. Documenting when the specimen was collected is important. Correct timing is also important, particularly in therapeutic drug monitoring. For instance, aminoglycoside levels cannot be interpreted appropriately without knowing whether the specimen was drawn just before (“trough” level) or after (“peak” level) drug administration. Drug levels cannot be interpreted if they are drawn during the drug’s distribution phase (eg, digoxin levels drawn during the first 6 hours after an oral dose). Another example is monitoring of unfractionated heparin (UFH) infusion, ie, specimen should be collected 6 hours after initiation of UFH infusion and 6 hours after any change in UFH infusion rate. Substances that have a circadian variation (eg, cortisol) can be interpreted only in the context of the time of day the sample was drawn. Testing for glucose-6-phosphate dehydrogenase (G6PD) deficiency is usually performed 2–3 months after a hemolytic episode, since G6PD enzyme activity may be spuriously normal shortly after acute hemolysis.

During specimen collection, certain principles and standard operative procedures must be followed. Standard blood collection devices and appropriate vacuum tubes (ie, those containing appropriate anticoagulant or gel separator or both for serum or plasma preparation) should be used. Specimens should not be drawn above an intravenous line or from an indwelling line because this may contaminate the sample with intravenous fluid and drug (eg, heparin). Excessive tourniquet time leads to hemoconcentration and an increased concentration of protein-bound substances such as calcium. Lysis of cells during collection of a blood specimen results in spuriously increased serum levels of substances concentrated in cells (eg, lactate dehydrogenase and potassium). Certain test specimens may require special handling, transport, processing, or storage conditions (eg, specimens for blood gas and serum cryoglobulin). Delay in delivery of specimens to the laboratory can result in ongoing cellular metabolism and therefore spurious results for some studies (eg, low serum glucose). Pre-analytical errors have been reported to account for 75% of testing errors, and they can be costly.

There is growing interest in point-of-care testing, for which specimen collection and handling are equally important. Point-of-care testing involves extensive quality assurance as well as training and competency evaluation of those at the site of care delivery (eg, staff in operating room, emergency department, intensive care unit, and outpatient clinics). The unit cost per test is high, through loss of economy of scale offered by automation in main laboratory, but there are potential benefits to patient care because of rapid delivery of results and also potential reduction of other facility costs.