Dear Alison,
You say :
“However, many partners feel that it is unacceptable to just guess the mid point of the arm when taking the measurement.”
I suggest you ask these partners what is the evidence in favour of the tradition of using the left arm only and exactly measure the mid point. My perception is that this has been uncritically repeated for years from article to article and from books to guidelines following a WHO document published in the 60’s. See:
Jelliffe Assessment nutritional status community
http://apps.who.int/iris/handle/10665/41780
Technique mesure MUAC : section 2 p 77
This made some sense in this document which dealt with overall assessment of the community, including adults. Among adults choosing the left arm may be warranted as the right arm may be bigger as a result of intensive physical activity. And it is important to get the mid point to get the biceps which may bulge at that place especially again among those engaged in heavy physical activity.
I am not aware this has been ever validated in children who are not engaged in heavy physical activity and have a rather tubular upper arm.
To my knowledge, the most extensive study using MUAC to detect high risk children was done in Bangladesh. It showed (and many other studies) that MUAC was very good at that. And in this study MUAC was measured by CHWs not checking mid-point. See:
https://www.ncbi.nlm.nih.gov/pubmed/2888951
I think the burden of proof is on the side of those who want the middle upper arm precisely checked. And until there is evidence supporting this approach, please do simplify screening by avoiding the ritual of measuring the upper arm mid-point.
I hope this helps