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Equations Developed to Determine Blindness, Limb Amputation Risks

Dec 4, 2015

Diabetes patients tend to overestimate complications risk, treatment benefits.

Researchers in the United Kingdom have developed a validated risk assessment equation to show the 10-year risk of blindness and lower limb amputation in diabetes patients. Microvascular and macrovascular complications are common in diabetes patients; risk factors included hyperglycemia and hypertension. Controlling these risk factors reduces the risk of these complications.


Data was collected via a prospective cohort study from general practitioners in England from 1998 to 2014. Development of the equations used data from over 400,000 diabetes patients, whereas validation used almost 350,000 diabetes patients. Data came from the UK QResearch database. Validation was carried out using that database as well as the Clinical Practice Research Datalink (CPRD) database.

The researchers looked at two outcomes: lower limb amputation and blindness. Blindness included one or both eyes, registered blindness, and severe visual impairment. They used a plethora of predictor variables, including ethnicity, presence of other disease states, age, body mass index, smoking status, length of diabetes diagnosis, and measures of glycemic control.

The algorithms are based on variables that patients are likely to know or that are routinely recorded in general practice computer systems. They can be used to identify patients at high risk for prevention or further assessment. Limitations include lack of formally adjudicated outcomes, information bias, and missing data.

These new algorithms calculate the absolute risk of developing these complications over a 10-year period in patients with diabetes, taking account of their individual risk factors.

The developed equations have C statistics values of at least 0.73, indicating good discrimination and calibration. These equations can help patients and providers concretely assess the risk of these life-changing complications. An accurate picture of the risk can help patients and providers make important clinical care decisions in the management of the disease state.

The algorithms are designed to provide better and more accurate information for patients and doctors on the absolute risks of blindness and amputation, to inform management decisions. Patients with diabetes tend to overestimate their risk of complications and also overestimate the benefits of treatment. For example, in one study, patients believed that they were 1.5 times more likely to become blind and 13 times more likely to have a lower leg amputation than estimates of absolute risk based on the DCCT trial.  Some people may argue that overestimating the risk of complications might result in patients being more likely to take intensive treatment. However, from a holistic and ethical point of view, more accurate individualized information on the risk of complications may help patients to make more informed decisions about the balance of risks and benefits of treatment options reflecting their own values and choices. Overestimation of the risk of complications might lead to increased levels of anxiety and depression, which could negatively affect quality of life. This is especially important as patients with diabetes are more likely than the general population to experience anxiety and depression.

For clinicians, complications could enable screening programs to be tailored to an individual’s level of risk and support the more rational use of scarce resources. For example, blindness can be prevented by screening for and treatment of retinopathy. Patients at high risk of blindness might need retinal screening more often than once a year. Those at higher risk of amputation might benefit from a proactive targeted program to prevent lower extremity amputation (including more frequent checks, tailored patient education, specially designed protective footwear, and early reporting of foot injuries), as this has been shown to substantially reduce the risk of emergency admissions, use of antibiotics, foot operations, and lower limb amputation compared with usual practice. Better information on the absolute risk of individual complications could also prompt more intensive treatment of modifiable risk factors—such as lowering of (HbA1c) and tighter blood pressure control—which are generally considered to reduce the risk of microvascular complications such as blindness.

See what your risk factors are. QDiabetes® (Amputation and blindness) equations.

Practice Pearls:

  • Researchers in the UK have developed algorithms to test the 10-year risk of blindness and lower limb amputation in diabetes patients.
  • Data came from hundreds of thousands of primary care patients.
  • The equations can be used to help patients and providers make better clinical decisions about care.

Hippisley-cox J, Coupland C. “Development and validation of risk prediction equations to estimate future risk of blindness and lower limb amputation in patients with diabetes: cohort study.” BMJ. 2015;351:h5441.