N-Methylhistamine, Urine

Test Mnemonic


CPT Codes

  • 82542 - QTY (1)


  • Methylhistamine-N, Urine
  • Methylhistamine-N, Urine
  • N-Methylhistamine, 24 Hour, Urine
  • N-Methylhistamine, 24 Hour, Urine
  • NMH
  • NMH


  • N-Methylhistamine, 24 Hr
  • Creatinine, 24 Hr Urine mg/24 h
  • Collection Duration
  • Urine Volume
  • Creatinine Concentration, 24 Hr Urine mg/dL

Performing Laboratory

Mayo Clinic Dpt of Lab Med & Pathology

Specimen Requirements

Volume Type Container Collect Temperature Transport Temperature Special Instructions
5 mLUrine, 24-hour (well-mixed)Clean containerRefrigerate during collection.RefrigeratedPlease include volume and hours of collection.

Minimum Specimen Requirements

Volume Type Container Collect Temperature Transport Temperature Special Instructions
3 mL     


Environmental Condition Description
Refrigerated28 days (preferred)
Ambient14 days
Frozen28 days

Days Performed

Tue, Thu

Turnaround Time

4 - 8 days


Name Description
Colorimetric Enzyme Assay 
Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) 

Reference Range

Special Info

Patient must not be taking monoamine oxidase inhibitors (MAOIs) or aminoguanidine as these medications increase N-methylhistamine (NMH) levels. Random urine collections are preferred for patients with episodic symptoms, for example in the context of allergic reactions, brought on by specific environmental factors. Please refer to N-Methylhistamine, Random, Urine (UMHISR).

Clinical Info

This test is useful for screening and monitoring mastocytosis and disorders of systemic mast-cell activation, such as anaphylaxis and other forms of severe systemic allergic reactions, using 24-hour urine collection specimens. Monitoring therapeutic progress in conditions that are associated with secondary, localized, low-grade persistent, mast-cell proliferation and activation such as interstitial cystitis. Cautions: While an average North American diet has no effect on urinary N-methylhistamine (NMH levels), mild elevations (approximately 30%) may be observed on very histamine-rich diets. This problem is more pronounced if random urine specimens are used and collected following a histamine-rich meal. NMH levels may be depressed in individuals who have an alteration in the histamine-N-methyl transferase gene, which encodes the enzyme that catalyzes NMH formation. This alteration results in an amino acid change that decreases the rate of NMH synthesis. When N-acetylcysteine is administered at levels sufficient to act as an antidote for the treatment of acetaminophen overdose, it may lead to falsely decreased creatinine results.