The 4-2-1 rule is a tool that doctors use to diagnose severe cases of diabetic retinopathy, an eye condition affecting people with diabetes that can progress to vision loss.

If high blood sugar levels damage the retina’s blood vessels, an eye condition called diabetic retinopathy may develop.

Diabetic retinopathy affects about 30% of people who have diabetes. About 9.6 million people in the United States have this condition. It’s more prevalent in males and non-Hispanic Black people.

Without treatment, damage to the retina resulting from diabetic retinopathy may lead to vision changes and loss of vision.

The retina, which is the thin tissue at the back of your eye near the optic nerve, senses light and details in what you see. It then sends this information to your brain.

Because there are usually no symptoms in the early stages of diabetic retinopathy, the National Eye Institute recommends annual dilated eye exams for people with diabetes. These exams allow a doctor to see any irregularities in the retina’s blood vessels.

There are four stages of diabetic retinopathy, in which damage to your retina progresses from mild to severe. The first few stages are called nonproliferative diabetic retinopathy (NPDR). The advanced stage is called proliferative diabetic retinopathy (PDR).

A diagnosis of severe NPDR is based on whether the tiny blood vessels in your retina meet the criteria in the 4-2-1 rule, which doctors use to determine the progression of diabetic retinopathy in the four quadrants of your retina.

The stages of diabetic retinopathy range from mild to advanced.

NPDR refers to the condition in its first four stages, while PDR is the most advanced stage.

Nonproliferative diabetic retinopathy

The four NPDR stages are:

  • Stage 1, mild: There may be some swelling of your retina’s tiny blood vessels, and they may begin to leak.
  • Stage 2, moderate: These blood vessels begin to develop blockages, with some leakage.
  • Stage 3, severe: Blockages spread and cut off the blood flow to parts of your retina, causing damage. A doctor will base this diagnosis on whether your retina meets one condition of the 4-2-1 rule.
  • Stage 4, very severe: A doctor will make this diagnosis if your retina meets two or more conditions of the 4-2-1 rule.

There’s a higher than 50% chance that severe NPDR will progress to PDR within 1 year. This chance is over 75% for very severe NPDR.

Proliferative diabetic retinopathy

PDR is the most advanced stage of diabetic retinopathy. It’s the most common cause of blindness in people 15–64 years old.

As your body tries to provide oxygen to your retina by boosting your blood circulation, a process called neovascularization happens. Tiny new blood vessels grow inside your retina and in the gel-like vitreous at the back of your eye.

PDR is divided into these three stages:

  • Stage 1, early: There are some new blood vessels in your retina.
  • Stage 2, high risk: There is additional neovascularization and possible vitreous bleeding (hemorrhage).
  • Stage 3, severe: Hemorrhages block a portion of the back surface of your eye. The macula, which is in the center of your retina and provides your central vision, may detach.

A doctor diagnoses severe NPDR based on the 4-2-1 rule, meaning one of the following applies to you:

  • Four quadrants of your retina have vessels with severe bleeding and tiny bulges called microaneurysms.
  • Two or more quadrants have veins with sections that are wider than usual (venous beading).
  • One or more quadrants have atypical blood vessels called intraretinal microvascular abnormalities growing.

If you have at least two of these, the diagnosis is very severe NPDR.

If the 4-2-1 rule applies and a doctor diagnoses severe NPDR, you’ll need to have regular dilated eye exams to check on the condition’s progression and lower your chance of vision loss.

Tell a doctor if you experience any changes in your vision.

The treatment for diabetic retinopathy depends on the severity of the condition. It may include:

Learn more about treating diabetic retinopathy.

Doctors use the 4-2-1 rule to diagnose severe NPDR. The rule is based on the extent of damage to the tiny blood vessels in the four quadrants of your retina.

It’s important to have regular dilated eye exams to monitor the progression of NPDR. Treatments to slow or stop its progress and prevent vision loss are available.