Can you measure plasma glucose with a regular glucose meter?
Excellent question... "Can you measure plasma glucose with a blood glucose meter?"

Absolutely Not! In my experience, if you try to measure plasma glucose with a whole blood glucose meter the results are highly variable for a number of reasons (mentioned below). Using blood glucose meters to measure plasma glucose is dangerous. The meters are calibrated for whole blood and not plasma, which is chemically different and will yield different results. Depending on the clinical situation (or experimental situation), the results can be significantly different and non-linear across a huge spectrum of concentrations. 

Here is a nice link from the University of Virginia demonstrating the basics of how one type of these meters works.

There are a number of types of glucose meters commercially available to the public as well as healthcare facilities. Most of these meters use varying solid-state chemistry techniques to drive the production of an electrical current that can be measured by the glucose meter. In other words, when the blood is taken up by the strip in the meter the glucose takes part in a redox chemical reaction that creates a small "electrical current" that is detected by the meter. The amount of glucose in the sample is proportional to the electrical current produced. However, there are a number of assumptions that the meters make (which can differ between meters). One of the most clinically significant assumptions is that the meter assumes that the hematocrit (i.e. the percentage of blood volume that is erythrocytes) is normal or approximately 45%. If you use these types of meters on plasma (which has no erythrocytes in it), the chemistry is inaccurate because the meter "thinks" all the glucose that it is detecting is actually dispersed within a pool of erythrocytes in a hematocrit of 45%. There are a number of other issues and important points discussed in this review.

When the clinical trials (e.g. NICE-SUGAR) began showing that intensive glucose control was associated with improved mortality, clinical practices changed from keeping blood glucose concentrations in critically ill patients between 150 and 250 mg/dl to a much lower concentrations (i.e. 80-120 mg/dl). As these new guidelines began rolling out, there were a number of subsequent studies that showed that intensive glucose control was actually associated with more frequent hypoglycemia episodes. There were likely a number of issues to explain this, but one of those was that if your meters were not correcting for differences in hematocrit the blood glucose meters would read spuriously higher than what the true measurement was (thus, you could be treating someone with insulin to lower their glucose that was 160 mg/dl - when in true the value was much, much lower).