What is the difference between blood sugar and blood pressure

Home monitoring of blood glucose and blood pressure levels can provide the patient and physician with valuable information for disease management. Because a variety of home monitoring devices are available, appropriate selection and use can be a daunting task. This article reviews devices for home monitoring of glucose and blood pressure and discusses their clinical utility.

Although there is no universal standard for accuracy of glucose meters, several groups have defined acceptable ranges.1–4 The U.S. Food and Drug Administration (FDA) requires glucose meters to produce self-monitoring results within 20 percent of a reference measurement but recommends results within 15 percent4; the FDA has stated that future meters should achieve results within 10 percent of reference at serum glucose concentrations of 30 to 400 mg per dL (1.7 to 22.2 mmol per L). The American Diabetes Association (ADA) recommends that meters produce readings within 5 percent of laboratory values.1 All meters currently on the market are considered to be clinically accurate in that they at least meet the FDA standard, although it is important to remember that they are not as accurate as a standard laboratory test.5,6 Given this broad range of possible error, making treatment decisions based solely on self-monitoring of blood glucose (SMBG) is not advised.

Glucose meters are most accurate when used properly.7 Thus, educating patients on proper use and what to do with the results is vital. Although the exact procedure for using a meter varies by product, potential pitfalls are similar. Common errors include poor maintenance (e.g., soiled meter), using expired test strips, obtaining an inadequate sample size, and failing to calibrate the meter.

CLINICAL UTILITY

Uses of SMBG data include identifying and treating hyper- and hypoglycemia; making decisions about food intake or medication adjustment when exercising; determining the effect of ingested food on blood glucose; and managing glucose fluctuations resulting from illness.8 Although the data are somewhat conflicting, larger, better-designed trials have shown that SMBG improves glycemic control when the results are used to adjust therapy.6 However, the data for reducing long-term complications are more conclusive for patients on insulin therapy.

Although the optimal frequency of monitoring is unknown, the ADA recommends SMBG three or more times a day for patients with type 1 diabetes.9 Patients with type 2 diabetes still benefit from at least periodic monitoring. Ultimately, the frequency and timing of SMBG should be determined by how the data will be used.8 SMBG can assist the patient and physician with adjusting diet and medications and maintaining appropriate glucose control. More frequent monitoring is beneficial during insulin dose adjustments.8 Postprandial monitoring is important to identify the effect of various foods on glucose levels and to monitor the effects of preprandial medications. Other factors, such as desire for tight control and current degree of control, will influence frequency of monitoring.

METER SELECTION

A number of glucose meters are available, with new models being released each year. Although home glucose meters use whole-blood samples, nearly all are plasma calibrated so that the results reflect plasma glucose. This allows home values to be compared directly to laboratory values. Glucose meters are largely differentiated based on their features. These include blood sample size required, test time, memory capability, ability to download results into data management software, and ability to perform alternate site testing (e.g., forearm). Meter selection should be based primarily on features desired by the patient (Table 1).

Newer technology has led to the development of continuous glucose meters that measure glucose in subcutaneous interstitial fluid and reflect changes relatively quickly. However, continuous monitors are not easily used on a long-term basis, and their current clinical utility is somewhat limited. Table 2 provides a comparison of commonly used glucose meters. Prices of most glucose meters and strips are comparable. However, patients often can purchase the meter for little or no cost after rebates. The long-term expenses come from the strips and other supplies. A few meters offer more advanced features, such as the ability to enter information (e.g., medication doses, carbohydrate intake, exercise) and voice prompts for the visually impaired.

Blood Pressure Monitoring

Electronic devices are available to measure blood pressure at the arm, wrist, or finger. Published data evaluating the accuracy of specific electronic blood pressure monitors are limited. However, several organizations have established standards for accuracy.10–13 Devices available in the United States that are known to meet these criteria are listed in Table 3. Clinical evaluations of wrist and finger devices have revealed that these instruments are considerably less accurate than their arm (brachial artery) counterparts, with finger monitors being the least accurate.14–16 Thus, arm monitors are preferred over wrist or finger monitors.

CLINICAL UTILITY

Office-based blood pressure measurements have been shown to result in higher values than those recorded at home.17 Studies have found variations of 9 to 23 mm Hg in systolic blood pressure and diastolic differences of 3 to 10 mm Hg.18 The most recent report from the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) defines home blood pressure values consistently greater than 135/85 mm Hg as hypertensive.19 The differences in systolic pressure between home and office blood pressure measurements increase with age and degree of anxiety during office visits (known as “white coat” hypertension). These differences also tend to be greater in men and in patients not taking antihypertensive medication.

A systematic review concluded that, compared with office monitoring, home monitoring is better at predicting target organ damage and cardiovascular mortality, predicting sustained hypertension in patients with borderline hypertension, and can be used to monitor drug effectiveness.17 This evidence is based on the use of validated blood pressure monitors. Although there are no evidence-based recommendations on frequency of home blood pressure measurements, it has been suggested that the minimal number of measurements to obtain an accurate assessment of a patient's usual blood pressure should be four times per day (twice in the morning and twice in the evening) for three consecutive days.17

MONITOR SELECTION

Although there are limited comparative data on specific blood pressure monitors, home monitoring offers several advantages in addition to its correlation with outcomes and drug effectiveness. It eliminates the white coat effect, allows for multiple readings, and may improve patient awareness and compliance with treatment. There are, however, a few limitations. Some home devices may not be appropriate in obese patients (because of limited cuff sizes), patients with arrhythmias or preeclampsia, and patients in whom vascular stiffening is suspected.17 In a recent randomized controlled trial, adjustment of antihypertensive medications based solely on home monitoring led to less-intensive drug treatment and poorer blood pressure control than usual care.20 Medication adjustments should incorporate values from home and office monitoring.

Electronic blood pressure models are relatively easy to use and display a digital readout. They may be semiautomatic (i.e., patient inflates and deflates cuff) or fully automatic (i.e., cuff inflates and deflates with the press of a button), although both types automatically measure the blood pressure. Electronic blood pressure readings correlate well with the auscultatory method.21 Despite the relative ease of using electronic blood pressure monitors, failure to follow protocol can lead to erroneous results. Table 4 outlines the recommended protocol for home blood pressure measurement.

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