The issue is not that it can't be used for anything, but that what you use it for depends on how much of whatever you're looking for will be present in any given volume of blood.

Sugar is present in sufficient quantities that the variation you get from one pinprick to another isn't going to be a big deal.

Many other things will be present in small enough concentrations that even if it is present in your blood you might not have sufficient quantities for it to be detectable in blood from a single pin-prick.