Preanalytics: stable 5 days at 2 - 8 °C to avoid bacterial growth the pH must be <5 1 day prior to and during collecting the 24 hour urine sample, following foodstuffs should be avoided: coffee, tea, alcohol, nicotine. Following drugs can impact the test results: alpha-methyldopa, barbiturates, beta- receptor antagonists, chlorpromazine, clonidine, guanethidine, insulin, reserpine, salicylic acid, tranquilizers, sulfonamide, tetracyclines. With the consent of the physician, the treatment with these drugs may be interrupted 7 days prior to starting the 24- hour urine collection.