For centuries, honey has been known to have broad-spectrum antimicrobial properties, and has been used to treat a variety of ailments. This medicinal value, which is exhibited by many types of honey, is believed to come from features such as honey’s acidity (low pH), high sugar concentration, and the presence of bacteriostatic and bactericidal compounds such as hydrogen peroxide, antioxidants, lysozyme, polyphenols, phenolic acids, flavonoids, and bee peptides. These features are sometimes collectively referred to as honey’s ‘peroxide’ activity.
Certain honeys derived from the New Zealand manuka tree (Leptospermum scoparium) have additional antimicrobial activity, above and beyond what is contributed by the above features.
For a number of years, the cause of this ‘non-peroxide activity’ (NPA) wasn’t well understood and the term ‘Unique Manuka Factor’ (UMF®) was adopted. Then, in 2006, scientists in Germany published research showing that manuka honey’s ‘non-peroxide’ antimicrobial activity (NPA) was closely related to the presence of the compound methylglyoxal (MG). The relationship between MG levels and NPA in New Zealand manuka honey was further demonstrated by researchers at Waikato University in a paper  that they published in 2008 (with correction in 2009). The graph below plots data from this paper, and shows the close relationship between MG and NPA levels (expressed as UMF® units).
The Source of Methylglyoxal in Manuka Honey
In 2009, scientists at the University of Waikato (Christopher Adams, Merilyn Manley-Harris and Peter Molan) published research  that showed that the methylglyoxal in New Zealand manuka honey originates from the chemical compound dihydroxyacetone (DHA), which is present in the nectar of manuka flowers to varying degrees. (Some manuka plants have more DHA in their nectar than others.)
This research found that “nectar washed from manuka flowers contained high levels of dihydroxyacetone and no detectable methylglyoxal.” Furthermore, “manuka honey, which was freshly produced by bees, contained low levels of methylglyoxal and high levels of dihydroxyacetone. Storage of these honeys at 37 degrees Celcius led to a decrease in the dihydroxyacetone content and a related increase in methylglyoxal.”
As the methylglyoxal (MG) in honey is created over time from the interaction of the dihydroxyacetone (DHA) in the honey with various naturally-occurring proteins and amino acids, the maximum concentration of MG in any particular sample of manuka honey can be indicated by its DHA concentration. All other things being equal, a manuka honey sample with a high DHA concentration has the potential to turn into a manuka honey with higher MG concentration than a comparable manuka honey sample with a low DHA concentration.
For this reason, Analytica Laboratories offers DHA and MG as part of the same test. One could think of the DHA as indicating potential MG concentration. However, both DHA and MG also naturally decrease over time, so there is a limit to how much a manuka honey’s MG concentration can increase - no matter how long it is stored.
 Adams, Christopher.J., Boult, C.H., Deadman, B.J., Farr, J.M., Grainger, M.N.C., Manley-Harris, M. , Snow, M.J. Isolation by HPLC and characterisation of bioactive fraction of New Zealand manuka (Letospermum scoparium) honey. J. Carbohydr. Res. 2008, 343, 651-659
 Adams, Christopher.J., Boult, C.H., Deadman, B.J., Farr, J.M., Grainger, M.N.C., Manley-Harris, M. , Snow, M.J. Corrigendum to "Isolation by HPLC and characterization of the bioactive fraction of New Zealand manuka (Leptospermum scoparium) honey"[Carbohydr. Res. 2008, 343, 651-659]. Carbohydr. Res. 2009, 344(18):2609-2609