Relying on their own perceptions of symptoms of hyperglycemia or hypoglycemia is usually unsatisfactory as mild to moderate hyperglycemia causes no obvious symptoms in nearly all patients. Other considerations include the fact that, while food takes several hours to be digested and absorbed, insulin administration can have glucose lowering effects for as little as 2 hours or 24 hours or more (depending on the nature of the insulin preparation used and individual patient reaction). In addition, the onset and duration of the effects of oral hypoglycemic agents vary from type to type and from patient to patient.

Personal (home) glucose monitoring
Control and outcomes of both types 1 and 2 diabetes may be improved by patients using home glucose meters to regularly measure their glucose levels. Glucose monitoring is both expensive (largely due to the cost of the consumable test strips) and requires significant commitment on the part of the patient. The effort and expense may be worthwhile for patients when they use the values to sensibly adjust food, exercise, and oral medications or insulin. These adjustments are generally made by the patients themselves following training by a clinician.

Regular blood testing, especially in type 1 diabetics, is helpful to keep adequate control of glucose levels and to reduce the chance of long term side effects of the disease. There are many (at least 20+) different types of blood monitoring devices available on the market today; not every meter suits all patients and it is a specific matter of choice for the patient, in consultation with a physician or other experienced professional, to find a meter that they personally find comfortable to use. The principle of the devices is virtually the same: a small blood sample is collected and measured. In one type of meter, the electrochemical, a small blood sample is produced by the patient using a lancet (a sterile pointed needle). The blood droplet is usually collected at the bottom of a test strip, while the other end is inserted in the glucose meter. This test strip contains various chemicals so that when the blood is applied, a small electrical charge is created between two contacts. This charge will vary depending on the glucose levels within the blood. In older glucose meters, the drop of blood is placed on top of a strip. A chemical reaction occurs and the strip changes color. The meter then measures the color of the strip optically.

Self-testing is clearly important in type I diabetes where the use of insulin therapy risks episodes of hypoglycaemia and home-testing allows for adjustment of dosage on each administration. However its benefit in type 2 diabetes is more controversial as there is much more variation in severity of type 2 cases. It has been suggested that some type 2 patients might do as well with home urine-testing alone. The best use of home blood-sugar monitoring is being researched.

Benefits of control and reduced hospital admission have been reported. However, patients on oral medication who do not self-adjust their drug dosage will miss many of the benefits of self-testing, and so it is questionable in this group. This is particularly so for patients taking monotherapy with metformin who are not at risk of hypoglycaemia. Regular 6 monthly laboratory testing of HbA1c (glycated haemoglobin) provides some assurance of longterm effective control and allows the adjustment of the patient's routine medication dosages in such cases. High frequency of self-testing in type 2 diabetes has not been shown to be associated with improved control. The argument is made, though, that type 2 patients with poor long term control despite home blood glucose monitoring, either have not had this integrated into their overall management, or are long overdue for tighter control by a switch from oral medication to injected insulin.

HbA1c test
A useful test that has usually been done in a laboratory is the measurement of blood HbA1c levels. This is the ratio of glycosylated hemoglobin in relation to the total hemoglobin. Persistent raised plasma glucose levels cause the proportion of these molecules to go up. This is a test that measures the average amount of diabetic control over a period originally thought to be about 3 months (the average red blood cell lifetime), but more recently thought to be more strongly weighted to the most recent 2 to 4 weeks. In the non-diabetic, the HbA1C level ranges from 4.0-6.0%; patients with diabetes mellitus who manage to keep their HbA1C level below 6.5% are considered to have good glycemic control. The HbA1c test is not appropriate if there has been changes to diet or treatment within shorter time periods than 6 weeks or there is disturbance of red cell aging (e.g. recent bleeding or hemolytic anemia) or a hemoglobinopathy (e.g. sickle cell disease). In such cases the alternative Fructosamine test is used to indicate average control in the preceding 2 to 3 weeks.

Ongoing monitoring
Recently, devices have been manufactured which provide ongoing monitoring of glucose levels on an automated basis during the day, for example:

   1. The Minimed Paradigm REAL-Time by Minimed, is a blood glucose monitoring device that provides blood glucose measurements to be made every five minutes. The patient can thus adjust an insulin infusion pump immediately and mimic the "feed-back" mechanism of a pancreas.
   2. The US Food and Drug Administration has also approved a non-invasive blood glucose monitoring device, the GlucoWatch G2 Biographer. This allows checking blood glucose levels, while puncturing the skin as little as twice a day. Once calibrated with a blood sample, it pulls body fluids from the skin using small electrical currents, taking six readings an hour for as long as thirteen hours. It has not proven to be reliable enough, or convenient enough to be used in lieu of conventional blood monitoring. Other non-invasive methods like radio waves, ultrasound and energy waves are also being tested.