Collect 24 hour urine specimen. Add 10 mL of 6 N HCl to container prior to start of collection. Following collection, mix well, measure 24 hour urine volume, aliquot 4 mL into a plastic urine container and refrigerate. Record total 24 hour urine volume and collection dates/times on request form. Specimens with no preservative are also acceptable: add HCl to container following collection, mix well and send 4 mL aliquot (preferred); alternatively a non-acidified aliquot will be accepted. Patient should void antihistamines, oral corticosteroids, and substances that block H2 receptors for at least 24 hours prior to specimen collection.