Class WO399 at The National Archives contains all surviving service records for members of Queen Alexandra's Military Nursing Service, the QAIMNS Reserve and the Territorial Force Nursing Service who served during the Great War. In all, there are 15,792 files in WO399, but there are a number of files missing from the run which were destroyed during the 'weeding' process of the early 1930s. At that time all the files were thinned out, and most now contain only a selection of papers, mainly relating to things such as pensions and gratuities.
But quite a number were totally destroyed - no-one really knows how many, or why certain files were chosen for destruction, while others survived. It's been suggested that there might be as many as 5,000 missing, but I find that difficult to believe, otherwise I would find that about 1 in 4 of those I look for are missing, which just isn't the case. I frequently draw a blank with service records, without understanding the reason, but recently I've failed to find a couple, which has proved quite helpful in one way, as it's highlighted one class of woman whose files are likely to have been scrapped. Both these women were members of the Territorial Force Nursing Service; they served during wartime, but were discharged from the service soon after the war; they had no entitlement to pension, and they both died during the 1920s from causes unrelated to their war service. As they were not going to serve at any time in the future, no pensions, and a non-attributable cause of death, it looks as though retention of their service files was considered unnecessary.
It will be interesting to try and pinpoint other groups of women whose files were similarly afflicted - finding a whole missing group would be a useful aid understanding the system.