Only a small specific proportion of manuka honey has the UMF healing property.

The reason why only some manuka honey has the unique UMF antibacterial property is not yet known. It could be from a subspecies of manuka or due to some environmental factor such as soil type. The researchers are currently looking into this question.

The presence of the UMF property can be detected only by scientific testing in a specially appointed laboratory. These tests determine the presence of and activity level of the special UMF property. To qualify to use the name UMF the honey must have a rating of UMF10 or more.

The UMF rating indicates the antibacterial strength of the honey after the honey has been packed in New Zealand (it does not apply to honey not packed in New Zealand).

Manuka Honey with high UMF activity is recognised by the Honey Research Unit at the University of Waikato as having special uses.