Interference by the 4-hydroxylated metabolite of propranolol with determination of metanephrines by the Pisano method

Measurements of urinary metanephrines by the Pisano procedure are unreliable in patients who are taking propranolol, owing to the presence of 4-hydroxypropranolol in the urine. Three properties of this propranolol metabolite lead to interference: (a) it is absorbed and eluted from ion-exchange resins under the conditions Pisano used for metanephrine isolation, (b) at high pH it absorbs at 350 to 360 nm, and (c) it is oxidized by periodate to a substance with negligible absorption in this region. Because 350 to 360 nm is the wavelength used to quantitate vanillin, the product formed from periodate oxidation of metanephrines, and because the unoxidized eluate is used as a specimen blank, the presence of 4-hydroxypropranolol spuriously decreases the measured urinary metanephrines; this has special significance because patients being tested for increased metanephrines are also likely to be receiving propranolol for hypertension.