Catecholamines in plasma are chemically labile and the specimens must be handled carefully, both because of rapid specific metabolism and rapid oxidation on exposure to air. For example, plasma-free norepinephrine has a half-life of approximately 2 minutes. To enhance accuracy, one must pay careful attention to the circumstances of specimen collection and to the preparation of the patient.
Many alterations in physiologic and pathologic states can profoundly affect catecholamine concentrations. Any environmental factor that may increase endogenous catecholamine production should be avoided. These include noise, stress, discomfort, body position, and the consumption of food, caffeinated beverages or nicotine. Caffeine and nicotine effects are short term, a few minutes to a few hours.
Other substances and drugs that may also affect the results include:
1. Substances which result in increased release or diminished metabolism of endogenous catecholamines.
●Monoamine oxidase inhibitors (MOI’s – a class of antidepressants with marked effects on catecholamine levels, particularly if the patient consumes tyrosine rich foods, such as nuts, bananas, or cheese)
●Catecholamine reuptake inhibitors including cocaine and synthetic cocaine derivatives, such as many local anesthetics, some of which are also antiarrhythmic drugs (eg, lidocaine)
●Some anesthetic gases, particularly halothane
●Withdrawal from sedative drugs, medical or recreational, in particular alcohol, benzodiazepines (eg, Valium), opioids and some central acting antihypertensive drugs, particularly clonidine, but, generally not cannabis or other hallucinogens such as lysergic acid diethylamide (LSD), mescal, or peyote.
●Vasodilating drugs (eg, calcium antagonists, alpha-blockers)
●Tricyclic antidepressants usually exert a negligible effect.
2. Substances that reduce or increase plasma volume acutely (eg, diuretics, radiographic contrast media, synthetic antidiuretic hormone [eg, desmopressin 1-deamino-8-d-arginine vasopressin, DDAVP])
3. Drugs which are metabolized to endogenous catecholamines. In the main this concerns Cardopa and L-dopa. These drugs are converted to dopamine, and dopamine measurements in patients taking these drugs will be artifactually elevated. Since isolated dopamine elevations are extremely rare, they should always be viewed with suspicion. A review of HPLC trace should be requested. On a careful review, our methodology usually, but not always, allows us to identify the unmetabolized parent drug, alongside dopamine.
Historically, a third category of potentially interfering substances was represented by molecules, which are either similar in chemical structure, antibody epitopes, or chromatographic migration pattern to the catecholamines, or have metabolites that can be mistaken for the catecholamines. The current HPLC-based assay is not subject to any significant direct interference of this kind. In particular, the following drugs, which used to be considered potential interferences, do not cause problems which cannot be resolved, in most cases, with the current assay: acetaminophen, allopurinol, amphetamines and its derivatives (methamphetamine, methylphenidate [Ritalin], fenfluramine, methylenedioxymethamphetamine [MDMA] [ecstaxy]), atropine, beta-blockers (atenolol, labetolol, metroprolol, sotalol), buspirone, butalbital, carbamezapine, chlorazepate, chlordiazepoxide, chlorpromazine, chlorothiazide, chlorthlidone, clonidine, codeine, diazepam, digoxin, dimethindene, diphenhydramine, diphenoxylate, dobutamine, doxycycline, ephedrine and pseudoephedrine, fludrocortisones, flurazepam, guanethidine, hydralazine, hydrochlorothiazide, hydroflumethiazide, indomethacin, insulin, isoprenaline, isosorbide dinitrate, L-Dopa, methenamine madelate (mandelic acid), methyldopa, methylprednisolone, nitrofuantioin, nitroglycerine, oxazepam, pentazocine, phenacetin, phenformin, pheonobarbital, phenytoin, prednisone, probenecid, progesterone, propoxyphene, propanolol, quinidine, spironolactone, tetracycline, thyroxine, and tripelennamine.
On occasion, when interference cannot be resolved an interference comment will be reported.
The variability associated with age, gender, and renal failure is uncertain.