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Patient should be fasting for four or more hours without smoking. Walnuts, bananas, and a-methyldopa (Aldomet) should be avoided for a week prior to specimen collection. Other drug interference may occur, including epinephrine and epinephrine-like drugs (eg, nosedrops, sinus and cough preparations, bronchodilators, appetite suppressants). Test is unreliable in subjects on levodopa or methenamine mandelate. Avoid patient stress.2 See Limitations. An indwelling heparinized venous catheter is advocated, since venipuncture can cause an increase in the substances for which testing is being done. Patient should remain supine in quiet surroundings for at least 30 minutes.
High-pressure liquid chromatography (HPLC)
Incorrect specimen; plasma not frozen; inadequate patient preparation