Here is a glucometer comparison chart from 2016
http://integrateddiabetes.com/wp-content/uploads/2016/09/meter-accuracy-Sept-2016.pdf

and a comparison study from 2016 Journal of Diabetes and Science
http://sites.bu.edu/bionicpancreas/files/2016/11/Comparative-Accuracy-Meters-Ekhlaspour-J-Diabetes-Sci-Technol-2016.pdf

I usually tell my human diabetic patients (I am a clinical pharmacist) "if its under 120 mg/dl you can probably rely on that +/- 10 mg/dl" (so a reading of 100 is actually 90-110)

Currently, I am trying to time a glucometer BS with a lab BS to compare results for my mare.  The lab sample must be spun and separated fairly quickly (<90 min) or the glucose result from the lab will be falsely low.

In my personal experience in a human hospital setting - even when the on-site lab tests multiple samples the BS can have a wide variety of numbers for just one patient.  It's a very small snapshot in time.

--
Eva and Anske
October 2017 Corvallis OR
Case History Anske 9yo Friesian Mare https://ecir.groups.io/g/CaseHistory/files/Eva%20and%20Anske