Presumptive drug testing can give you rapid results that can be used to help improve patient care. But whether a presumptive test is right for your patient depends on many factors, according to Dr Leland McClure, Director, Medical Science Liaison, Medical Affairs at Quest Diagnostics, and understanding the complexities of presumptive testing is vital for making the right testing choice. Dr McClure outlined these complexities in a recent webinar.
“A presumptive test is a qualitative test used to detect the possible presence of a drug or drug class,” he explained, based on the definition issued by the American Medical Association in 2015. “Definitive” testing is a qualitative or quantitative procedure that identifies the specific drugs and associated metabolites. Definitive testing typically follows presumptive testing, or may be the test of first choice.
======
CPT codes for presumptive testing1
CPT code: 80305 Drug test(s), presumptive, any number of drug classes, qualitative; any number of devices or procedures, (eg, immunoassay) capable of being read by direct optical observation only (eg, dipstick, cups, cards, cartridges) includes sample validation when performed, per date of service (maps to 80300 or G0477).
CPT code: 80306 Drug test(s), presumptive, any number of drug classes, qualitative; any number of devices or procedures, (eg, immunoassay) read by instrumented assisted direct optical observation (eg, dipstick, cups, cards, cartridges) includes sample validation when performed, per date of service (maps to 80300 or G0478).
CPT code: 80307 Drug test(s), presumptive, any number of drug classes, qualitative; any number of devices or procedures, by instrument chemistry and analyzers (eg, utilizing immunoassay [EIA, ELISA, EMIT, FPIA, IA, KIMS, RIA]), chromatography (eg, GC, HPLC), and mass spectrometry either with or without chromatography, (DAT, DESI, GC-MS, GC-MS/MS, LC-MS, LC-MS/MS, LDTD, MALDI, TOF) includes sample validation when performed, per date of service (maps to 80301, 80302, 80303, 80304 and G0479).
Most presumptive drug tests at Quest Diagnostics will fit the CPT code 80307.
======
An example of a presumptive drug test is the enzyme multiplied immunoassay, in which the assay reagents include an antibody to the drug and an enzyme-labeled drug molecule of the same drug that is being tested. When the antibody binds to the enzyme-labeled drug, the enzyme is inactivated. When the patient specimen contains the targeted drug, the patient drug competes with the enzyme-labeled drug for antibody binding. The unbound enzyme-labeled drug molecule is active and catalyzes a change in another reagent, giving a color change. The degree of color change is proportional (or in some cases, inversely proportional) to the amount of drug which may be present in the specimen.
Immunoassay drug tests may be targeted for a single compound, or for a whole class. “The assay specificity decreases as the scope of drugs or metabolites detected is increased,” said Dr McClure. “This is a critical point,” one that affects how results may be interpreted.
Chemistry instruments that perform these drug tests are calibrated to the sensitivity and linear range of the assay, and provide an objective result. Test results read by the human eye, such as point-of-care colorimetric assays, are subjective. The analyst must be trained in performing and interpreting point of care results, “and should be tested for colorblindness,” Dr McClure noted.
When performed and interpreted properly, “presumptive tests provide a rapid and reliable method that can be used to distinguish negative from non-negative results,” he said. A negative result is one that is less than the predetermined cut-off value, and a non-negative one is equal to or greater than the cutoff. Non-negative results may include true positive and false positive results.
False positive immunoassay results may arise from non-target interference. For instance, the Parkinson’s disease drug amantadine may give false positive results for amphetamines, while the antipsychotic chlorpromazine and the antihistamine diphenhydramine may give false positive results for methadone.
Confounding factors can interfere with a clear interpretation of the result. First, the linear range of drug-testing immunoassays is limited, Dr McClure noted, with a flattening of the response curve at both ends of the range. If the specimen concentration falls at either end, the results may be skewed. Paradoxically, an extremely high specimen concentration can give a negative result, called the “hook effect,” due to depletion of the colorimetric substrate.
Patients can vary in their pharmacogenomic ability to metabolize drugs, and may exhibit both very rapid and very slow metabolic gene variants in the population. And of course, the specimen must be collected in a manner to control for patient actions of alteration or substitution.
“And we must always keep in mind that the urine drug concentration does not reflect the amount of drug in the patient’s body, the dose, or when the drug was taken or in what form,” Dr McClure cautioned.
“Definitive drug testing is complementary analysis used to rule out false positive results,” Dr McClure said. “Positive presumptive results are typically followed with definitive testing to identify the specific drugs in the specimen.” Test options can be ordered that include reflex definitive testing to confirm presumptive positive test results. Where presumptive testing may be insensitive, definitive testing may be used to rule out false negative presumptive results.
Published date: March 6, 2019
