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Christopher Saudek, M.D.
02-03-2011
- With self-monitoring of blood glucose (SMBG), the patient measures their own blood glucose level using a drop of blood applied to a reagent stick.
- A meter displays the blood glucose level at the moment it is tested.
- Wide variety of different meters and strips, each with strengths and weaknesses.
- Usually, a small drop of blood is taken from a finger prick, although some meters can use blood from alternate sites such as forearm.
- Results are usually stored in meter by date, time and result, and can be downloaded with simple software packages; some meters display their results on an external insulin pump ("smart pumps").
- Either glucose oxidase or glucose dehydrogenase enzymes are impregnated on test strips, with measurement of hydrogen peroxide or electron production.
- Accuracy highly technique-dependent, ideally +10%, but generally about +10-15% accurate, compared to labratory values.
- Most meters calibrated to display result as plasma glucose, although testing whole blood.
- SMBG is not as accurate as laboratory plasma glucose but far more useful since it is done repeatedly, at home.
- Strips must also be shipped and stored properly, and some meters require entry of a code on the vial.
- Uniformly agreed that people with type 1 diabetes should self-monitor regularly and frequently (i.e. several times daily)
- Evidence of benefit of regular SMBG in all insulin-requiring diabetes, either type 1 or 2 diabetes (Soumerai)
- Controversy over cost-effectiveness of using SMBG in non-insulin-requiring type 2 diabetes
- In non-insulin requiring type 2 diabetes, a meta-analysis found modest (-0.16%) improvement in A1c (St. John)
- Health care professional must evaluate results and provide feedback to the patient.
- Patients must understand the meaning of results, and how to respond to high or low results.
- Interpretation greatly facilitated by downloading results with convenient, graphic displays.
- SMBG can confirm whether symptoms are caused by high or low blood glucose or whether a change in treatment is effective.
- Controversial whether or when routine pre-meal or post-meal SMBG is most useful.
- User error is the most common cause of serious inaccuracy (Bergenstal).
- Strips may be defective upon purchase or become defective if improperly stored in extreme temperature or humidity.
- Less accurate at low (< 50 mg/dl) or high (> 300 mg/dl) glucose levels, but accuracy even at extremes is adequate.
- Accuracy also requires that hands be dry and clean of any contact with sugar, and that drop of blood be adequate volume.
- Many meters require calibration and/or inputting a code for vial of strips into meter.
- Unusually, interference can occur. Glucose oxidase strips may have interference from high-dose acetaminophen, salicylates, ascorbic acid or low oxygen. Glucose dehydrogenase strips may be affected by maltose or galactose sometimes used on inpatient settings or peritoneal dialysis.
- Regular use of SMBG helps diabetes self-care only if results are translated into changes in treatment.
- SMBG empowers patients to take part in their own self-care, to know when their glucose level is high or low.
- We encourage people with type 1 to monitor at least 2-4 times daily.
- With type 2 diabetes, we recommend SMBG depending on instability of glucose levels and treatment changes.
- Particularly recommend SMBG in T2DM if A1c is unexpectedly high, or there is a change in treatment.
- We usually recommend pre-meal and bedtime testing, except for suspected isolated post-meal hyperglycemia.
- Profiling various times of day rather than only one time can be helpful.
- If SMBG readings do not correlate with A1c, consider confounders, more frequent testing (including post-meal or nocturnal testing) or continuous glucose monitoring.
- Strongly recommend reviewing downloads of glucometer data, which easily display summary statistics (e.g. average and standard deviation of glucose; number of tests performed per day; patterns over day, week or month; frequency and timing of serious highs or lows).
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