How accurate are glucose monitors?

Glucose monitors are quite accurate in that any given reading on the same sample of blood should only vary by a  few percentage points  (i.e., 5–10).  In  order  to obtain approval from the Food and Drug Administra- tion, such medical devices have to show that they are

both precise and accurate. This means that  they will

obtain a similar result under similar conditions and that the result will be close to the real true value, as best as we are able to ascertain it, i.e., according to the accepted gold standard, which is glucose measured in the labora- tory. Furthermore, when such a device is compared to the gold standard measurement in clinical trials, a spec- ified number of readings have to fall very close to the readings obtained  by the  gold standard  device. This value is more than 95%. The rules for approval of such devices have  been  published  by  the  FDA  and  can be viewed at  http://www.fda.gov/cdrh/oivd/guidance/

1171.pdf. Although  users are often surprised by the variation in results that they will see upon obtaining a reading within a very short space of time from a prior reading, it has to be acknowledged that the variation is seldom such that a different course of action would be taken. For example, insulin would be given or not given or a low blood sugar level would be treated  or not treated.  Actual  numerical  accuracy tends  to  be  less when the glucose level is very low. However, since such a low value should  always be treated, the accuracy of the actual reported number is not critical.

When a surprising glucose value that does not fit with the  expected circumstances is obtained,  it  is always prudent to recheck that reading, on either the same or a different monitor. Also, it is advisable to check that the monitor is functioning  well, by checking the low battery indicator, assuring that the correct coding has been entered and that the test strips are  within date and inserted properly. Sometimes, variation in  tech- nique by the user accounts, at least partially, for varia- tion in the result obtained. This can include liquid on the puncture  site, contamination with glucose, exces- sive squeezing of the finger, and sweating.