This article is from the Diabetes FAQ, by Edward Reid email@example.com with numerous contributions by others.
bG (blood glucose) meters are not as accurate as the readings you get from
them imply. For example, you might think that 108 means 108 mg/dl, not 107 or
109. But in fact all meters made for home use have at least a 10-15% error
under ideal conditions. Thus you should interpret "108" as "probably between
100 and 120". (Similar considerations apply if you measure in units of
mmol/L.) This is a random error and will not be consistent from one
determination to the next. You cannot expect to get exactly the same reading
from two checks done one after the other, nor from two meters using the same
This is generally considered acceptable because variations in this range will
not make a major difference in treatment decisions. For example, the
difference between 100 and 120 may make no difference in how you treat
yourself, or at most might make a difference of one unit of insulin. With
present technology, more accurate meters would be much more expensive. This
expense is only justified in research work, where such accuracy might detect
small trends which could go undetected with less accurate measurements.
This discussion applies to ideal conditions. The error may be increased by
poor or missing calibration, temperatures outside the intended range,
outdated strips, improper technique, poor timing, insufficient sample size,
contamination, and probably other factors. Contamination is especially
serious since it can happen so easily and is likely to result in an overdose
of insulin. Glucose is found in fruits, juices, sodas, and many other foods.
Even a smidgen can seriously alter a reading.
When comparing meter readings with lab results, also note that plasma readings
are 15% higher than whole blood, and that capillary blood gives different
readings from venous blood.
Visually read strips are slightly less accurate than meters, with an error
rate around 20-25%.
For some meters, strips are available from manufacturers other than the meter
manufacturer. Some m.h.d. readers have compared the strips side-by-side and
found those from one manufacturer to read consistently lower than the strips
from another. The differences are not likely to make a significant difference
in your treatment, but are large enough to be noticeable and possibly
confusing. For this reason it is not a good idea to change strip
manufacturers without comparing the readings from one with the readings from
I've seen no such direct comparison of meters, but the possibility exists that
some meters might read consistently lower than others. Be careful when
By "error rate" I mean twice the standard deviation from the mean. An error
rate of 15% says that about 95% of the readings will be within 15% of the