N-Methylhistamine, 24 hour urine

Alphabetical Test listing

N-Methylhistamine, 24 hour urine-994

N-Methylhistamine, 24 hour urine
Histamine metabolites
MAST cell disease
Urinary N-Methylhistamine
  • Screening for and monitoring of 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



Patient must not be taking monoamine oxidase inhibitors (MAOIs) or aminoguanidine as these medications increase N-methylhistamine (NMH) levels

Urine, 24 hr
5 mL
3.0 mL

How to collect a 24 hr urine sample

Application of temperature controls must occur within 4 hours of completion of the collection.


  1. Measure and record total volume
  2. Mix well
  3. Transfer 5 mL (minimum 3.0 mL) to a Screw cap transfer vial/tube (Mayo T465)



Refrigerated (preferred) - 28 days

Frozen - 28 days

Ambient - 28 days

Mayo Clinic Laboratories (NMH24): R-NX
Tu, Th 10am
3 - 7 days

NMH1D: Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

CRT24: Enzymatic Colorimetric Assay



0 - 5 years:  120 - 510 µg/g creatinine
6 - 16 years:  70 - 330 µg/g creatinine
> 16 years:    30 - 200 µg/g creatinine


While an average North American diet has no effect on urinary N-methylhistamine (NMH) levels, mild elevations (around 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.