Chapter adapted and updated, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.
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. For certain test supplement should be avoided before testing (eg, biotin supplement should be avoided for 24–36 hours before thyroid function testing).Patients should rest for 15 minutes or more, either lying or sitting before blood specimen collection.
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. Hemolyzed specimen can be a problem because lysis of cells during blood sample collection results in spuriously increased serum levels of substances concentrated in cells (eg, LD 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 ...