Evidence of a strong association between frequency of self-monitoring of blood glucose and hemoglobin A1c levels in T1D exchange clinic registry participants.

OBJECTIVE: Despite substantial evidence of the benefit of frequent self-monitoring of blood glucose (SMBG) in type 1 diabetes, certain insurers limit the number of test strips that they will provide. The large database of the T1D Exchange clinic registry provided an opportunity to evaluate the relationship between the number of SMBG measurements per day and HbA1c levels across a wide age range of children and adults. RESEARCH DESIGN AND METHODS: The analysis included 20,555 participants in the T1D Exchange clinic registry with type 1 diabetes ≥1 year and not using a continuous glucose monitor (11,641 younger than age 18 years and 8,914 18 years old or older). General linear models were used to assess the association between the number of SMBG measurements and HbA1c levels after adjusting for potential confounding variables.
RESULTS: A higher number of SMBG measurements per day were associated with non-Hispanic white race, insurance coverage, higher household income, and use of an insulin pump for insulin delivery (P < 0.001 for each factor). After adjusting for these factors, a higher number of SMBG measurements per day was strongly associated with a lower HbA1c level (adjusted P < 0.001), with the association being present in all age-groups and in both insulin pump and injection users.
CONCLUSIONS: There is a strong association between higher SMBG frequency and lower HbA1c levels. It is important for insurers to consider that reducing restrictions on the number of test strips provided per month may lead to improved glycemic control for some patients with type 1 diabetes.

The advent in the 1980s of meters for self-monitoring of blood glucose (SMBG) has had a substantial impact on the management of type 1 diabetes (1). Several studies have demonstrated a strong correlation between frequency of SMBG and glycemic control (2–5). However, acceptance of the value of frequent SMBG has not been universal and many insurers limit the number of test strips that they will provide to four to six strips per day. In the past year, the Washington State Healthcare Authority questioned whether sufficient evidence is available to justify unlimited coverage of SMBG test strips for patients with type 1 diabetes (6).

The large database of the T1D Exchange clinic registry provided an opportunity to evaluate the relationship between the number of SMBG measurements per day and HbA1c across a wide age range of children and adults, and to evaluate factors associated with the number of SMBG measurements per day.

RESEARCH DESIGN AND METHODS

The T1D Exchange Clinic Network includes 67 pediatric and adult endocrinology practices based in the United States. A registry of individuals with type 1 diabetes commenced enrollment in September 2010 (7). Each clinic received approval from an Institutional Review Board. Informed consent was obtained according to Institutional Review Board requirements from adult participants and parents/guardians of minors; assent from minors was obtained as required. Data were collected for the registry’s central database from the participant’s medical record and by having the participant or parent complete a comprehensive questionnaire, as previously described (7).

This report includes data of 20,555 participants enrolled through 1 August 2012 who met the following criteria: type 1 diabetes for at least 1 year; not pregnant; not using real-time continuous glucose monitoring; and availability of an HbA1c measurement between 6 months before and 1 month after enrollment.

Information on SMBG measurements per day was obtained on a questionnaire completed by participants 18 years old or older, parent or guardian of participants younger than 13 years old, and by either the participant or the parent/guardian for participants 13 years old to younger than 18 years old, with the following question: Approximately how many times per day are you (is your child) checking your (his/her) blood glucose with a blood glucose meter? For a subset of the participants, the number of SMBG measurements per day was available from a meter download located in the clinic chart. HbA1c levels, mainly measured with point-of-care devices (74% DCA, 4% from another point-of-care device, 19% from a laboratory, 3% by an unrecorded method), were obtained from the clinic chart. When more than one HbA1c value was available between 6 months before and 1 month after enrollment, the value obtained closest to the enrollment date was used.

Statistical methods

Frequency of SMBG measurements per day was categorized for illustration purposes into five groupings: 0–2 times per day; 3–4 times per day; 5–6 times per day; 7–9 times per day; and ≥10 times per day. Analyses stratified by age used the following age-groups: 1 to younger than 6 years old; 6 to younger than 13 years old; 13 to younger than 18 years old; 18 to younger than 26 years old; 26 to younger than 50 years old; 50 to younger than 65 years old; and 65 years or older.

Demographic and clinical factors associated with the number of SMBG measurements per day were assessed in linear regression models adjusted for age-group. Factors with a significance level ≤0.05 from individual factor models adjusted for age were included in the initial multivariate linear regression model and were removed from the final model if adjusted P ≥ 0.01. General linear models were used to assess the association between the number of SMBG measurements per day and HbA1c in each age-group after adjusting for potential confounding variables. Additional analyses assessing the association between frequency of SMBG per day and HbA1c <7.0% were performed using logistic regression. Covariates adjusted for in the multivariate models included the following: sex; race/ethnicity; insulin delivery method; insurance status (private, other, none); and household income (participants who were living alone but still supported by parents were asked to estimate their family income). The effect of the interaction between SMBG and household income on HbA1c levels was evaluated. Separate analyses for pump and injection users also were performed. The Van der Waerden normal scores of the frequency of SMBG per day were used in the models as a result of the skewed distribution of the data.

Self-reported SMBG measurements per day were used in the analyses. Analytic results were similar when data from clinic meter downloads were used from the subset of participants for whom downloaded data were available (data not shown). Analyses were conducted using SAS software version 9.3 (SAS Institute, Cary, NC). All P values are two-sided. In view of the large sample size and multiple comparisons, only P < 0.01 was considered statistically significant.

RESULTS

The cohort included 20,555 participants: 11,641 younger than 18 years old and 8,914 who were 18 years old or older. Characteristics of the cohort are shown in Table 1. Mean number of SMBG measurements per day was lower among participants 13 to younger than 26 years old (4.9 ± 2.2) than among younger (6.7 ± 2.3) and older participants (5.3 ± 2.5; P < 0.001; Table 2). Among 10,384 participants for whom a meter download was available, self-reported SMBG measurements averaged 5.7 ± 2.5 per day compared with the clinic assessment from meter downloads of 4.8 ± 2.8 per day (Pearson correlation = 0.65).

Table 1

Descriptive characteristics of the cohort

Table 2

SMBG by age-group

In a multivariate model adjusted for age-group, participants who reported a higher number of SMBG measurements per day were more likely to be non-Hispanic white, have private insurance, have higher household income, and use an insulin pump for insulin delivery (Supplementary Table 1; P < 0.001 for each factor).

A higher number of SMBG measurements per day was strongly associated with a lower HbA1c in all age-groups (Fig. 1 adjusted means and Table 3 unadjusted means; P = 0.002 for 1 to younger than 6 years and P < 0.001 for all other age-groups adjusted for potential demographic and socioeconomic confounders) despite the differences in HbA1c between age-groups. The association was present in both insulin pump and injection users (Fig. 1 adjusted means; P < 0.001) and across annual household income categories (Supplementary Table 2). There was no significant interaction between SMBG and household income on HbA1c levels for any age-group. The association between SMBG and HbA1c levels appeared to level-off at approximately 10 SMBG measurements per day, with adjusted mean HbA1c being similar in participants testing 10–12 times as in those testing ≥13 times per day, 7.8 and 7.7%, respectively. Results were similar when evaluating the association between SMBG measurements per day and HbA1c <7.0% (Table 3).

Figure 1

A: Association between frequency of SMBG per day and HbA1c. Solid black line and diamonds represent those 1 to younger than 13 years old. Solid black line and squares represent those 13 to younger than 26 years old. Solid black line and triangle represent those 26 to younger than 50 years old. Dotted black line and squares represent those 50 years old and older. HbA1c means with numbers <30 are not included here. Means are adjusted for insulin delivery method, sex, race/ethnicity, insurance status, and household income (treated as ordinal variables with a missing indicator). B: Association between frequency of SMBG per day and HbA1c among insulin pump users. Solid black line and diamonds indicate those 1 to younger than 13 years old. Solid black line and squares indicate those 13 to younger than 26 years old. Solid black line and triangle indicate those 26 to younger than 50 years old. Dotted black line and squares indicate those 50 years old or older. HbA1c means with numbers <30 are not included here. Means are adjusted for sex, race/ethnicity, insurance status, and household income (treated as ordinal variables with a missing indicator). C: Association between frequency of SMBG per day and HbA1c among injection users. Solid black line and diamonds represent those 1 to younger than 13 years old. Solid black line and squares represent those 13 to younger than 26 years old. Solid black line and triangle represent those 26 to younger than 50 years old. Dotted black line and squares represent those 50 years old and older. HbA1c means with numbers <30 are not included here. Means are adjusted for sex, race/ethnicity, insurance status, and household income (treated as ordinal variables with a missing indicator).

Table 3

Association between frequency of SMBG per day and HbA1c according to age

CONCLUSIONS

SMBG is the cornerstone of modern-day therapy for people with type 1 diabetes. Early studies clearly demonstrated that capillary glucose information was valuable for making appropriate decisions regarding insulin dosing and therefore for improvement of diabetes control (1). The intensive therapy group in the Diabetes Control and Complications Trial (DCCT) was asked, as part of their therapy, to perform SMBG before meals and at bedtime as well as overnight once per week (8). Whereas this was not a randomized trial for SMBG, it is, to our knowledge, the last study comparing a treatment regimen including glucose testing four times daily against little to no testing (9). Today, it would be impossible to perform a randomized trial in type 1 diabetes comparing SMBG with no SMBG given our understanding of the importance of glucose control in preventing the complications of type 1 diabetes (10). The best alternative is to use a large database, such as the T1D Exchange clinic registry, to analyze associations between frequency of SMBG and HbA1c and to provide the evidence desired by payers, such as the State of Washington, to support the cost-effectiveness of providing coverage for test strips.

Consistent with other smaller studies in the United States and the large Germany and Austria DPV registry (2–5), we clearly show that for all ages and with both major forms of insulin delivery (pump and multiple injections), increased frequency of SMBG is associated with lower mean HbA1c. This is true even after adjusting for demographic and socioeconomic confounders. Our study observed a similar association between SMBG and HbA1c across levels of household income, which has not been previously reported. Nevertheless, because diabetes management in those with more frequent SMBG likely differs from those with less frequent SMBG, frequent SMBG by itself is not the sole explanation for the association with lower HbA1c, but it almost certainly is an important contributor. Of course, for frequent SMBG to influence HbA1c, the blood glucose information must be used effectively in diabetes management including insulin dosing and meal and snack composition; the act of performing SMBG alone is not directly related to improvements in HbA1c. Thus, frequent SMBG is a behavior associated with good glycemic control but in itself does not have a direct causal relationship with glycemic control.

The 2012 American Diabetes Association clinical guidelines recommend SMBG at least three times per day for patients using insulin pump therapy or multiple insulin injections (11). In this analysis of individuals with type 1 diabetes, participants testing 3–4 times per day had a mean HbA1c of 8.6% compared with an HbA1c of 7.6% among those testing ≥10 times per day. Because prospective trials testing how the frequency of SMBG impacting HbA1c are not likely, we are hopeful that future guidelines better-reflect our current understanding of the relationship of SMBG and HbA1c.

Our data suggest a slight over-reporting of the frequency of SMBG compared with meter downloads, which could, in part, be explained by incomplete data for patients who use more than one meter and difficulty in interpreting downloaded meter data when the date of the meter is incorrect. However, these issues are not germane to our results, because the same association between frequency of SMBG and HbA1c is seen when the meter download glucose values were used in the analyses (data not shown).

In conclusion, there is a strong association between a higher SMBG frequency and lower HbA1c across the entire age range in our large population of patients with type 1 diabetes, with similar findings in pump users and injection users. The observational nature of the study precludes a definitive statement regarding causality. Nevertheless, it is important for insurers to consider that reducing restrictions on the number of test strips provided per month may lead to improved glycemic control for some patients with type 1 diabetes, resulting in a potential cost-savings from both short-term and long-term complications.