Specimen Types

Serum and plasma are the most common type of specimen used in TDM. These specimens may be interfered by all four types of interferents covered in this chapter. Additionally, collection tube additives for plasma or whole blood specimens, such as ethylene-diamino-tetra-acetate (EDTA), heparin, citrate, fluoride, and oxalate may chelate metal ions and thus interfere with label enzymes such as alkaline phosphatase and thus generating false-positive or false-negative results.

Whole blood specimens must be used for most of the immunosuppressant drugs. For such samples, hematocrit in addition to the main four sources of interference can also affect the assay results. Thus, false-positive tacrolimus results were reported in a microparticle enzyme immunoassay (MEIA) for patients with low hematocrit values and high imprecision at tacrolimus concentration <9ng/mL (4). The enzyme-multiplied immunoassay technique (EMIT assay) for tacrolimus was not affected. When the authors divided the study specimens in three groups by hematocrit percentage (<25, 25-35, and >35%), the difference between MEIA and EMIT assays increased as hematocrit percentage decreased. Moreover, false-positive results were reported in 63% of specimens with MEIA where patients did not receive any tacrolimus but only 2.2% of specimens using EMIT. Such false-positive values in the MEIA and EMIT methods ranged up to 3.7 and 1.3 ng/mL, respectively.

Urine is the most commonly used specimen for DAU testing. Urine samples less frequently contain measurable amounts of hemoglobin and bilirubin to cause measurable interference with an assay. Turbidity interference is possible in urine, but the cause is most likely bacterial growth. Preservatives in urine, such as acetic acid, boric acid, or alkali may interfere in some urine assays.

Cerebrospinal fluid (CSF) specimens are rarely used in TDM or DAU. Most common CSF interferent is hemolysis. Interference from turbidity is also possible in such specimens.

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