Eye doctors order blood work when something they see during an exam suggests a problem that goes beyond the eyes themselves. Your eyes are one of the few places in the body where blood vessels and tissue can be observed directly, making them a window into conditions like diabetes, autoimmune disorders, blood clotting problems, and infections. When your eye doctor spots certain patterns of inflammation, blood vessel damage, or swelling, blood tests help pinpoint the underlying cause so treatment can start quickly.
Signs of Diabetes or Poor Blood Sugar Control
One of the most common reasons an eye doctor orders blood work is diabetic eye disease. During a dilated exam, your doctor can see tiny blood vessels in the retina that are leaking, swollen, or growing abnormally. These changes, called diabetic retinopathy, are strongly tied to long-term blood sugar levels. The key blood test here measures HbA1c, which reflects your average blood sugar over the past two to three months. Most guidelines recommend keeping HbA1c below 7.0% to reduce the risk of eye damage.
If your eye doctor sees retinal changes consistent with diabetes and you haven’t been diagnosed, blood work can confirm it. If you already have diabetes, an HbA1c result tells your eye doctor how well your blood sugar has been controlled and helps predict whether your eye disease is likely to worsen. There’s a strong correlation between higher HbA1c values and more severe retinopathy.
Unexplained Eye Inflammation
Uveitis, an inflammation inside the eye, is one of the biggest triggers for blood work from an eye doctor. Uveitis causes pain, redness, light sensitivity, and blurred vision, and in many cases it’s driven by a systemic condition rather than a problem with the eye alone. About 40% of autoimmune-related eye cases in research involve uveitis, and another 30% involve inflammation of the blood vessels in the retina.
The specific tests your doctor orders depend on the type and location of inflammation. For the most common form, anterior uveitis (inflammation at the front of the eye), the American Academy of Ophthalmology recommends testing for HLA-B27, a genetic marker found in 50 to 80% of people with this condition compared to just 5% of the general population. Syphilis screening is also standard, since ocular syphilis can mimic many forms of eye inflammation and is treatable once identified.
Your doctor will also typically check for sarcoidosis and tuberculosis. Sarcoidosis, a condition that causes clusters of inflammatory cells in various organs, is screened with two blood markers: ACE (angiotensin-converting enzyme) and lysozyme. Lysozyme picks up about 84% of sarcoidosis cases, while ACE catches only about 27% but is highly specific, meaning a positive result strongly suggests the disease. Interestingly, some commonly ordered tests like ANA (antinuclear antibodies) have very little value in diagnosing uveitis in adults, with a positive predictive value of just 1%. A good eye doctor will skip those unless you have other specific findings like severe dry eye or joint inflammation.
Blood Vessel Blockages in the Retina
If your eye doctor finds a retinal vein occlusion, essentially a blood clot blocking a vein in the back of your eye, blood work becomes important for understanding why it happened. Everyone with this diagnosis gets tested for blood sugar and cholesterol levels, since diabetes and high cholesterol are major risk factors.
If you’re under 50, have clots in both eyes, or have a personal or family history of blood clots, the workup goes much deeper. Your doctor will order a full thrombophilia screen, which checks for inherited clotting disorders like Factor V Leiden mutation and protein C or protein S deficiency. They’ll also test for lupus anticoagulant and homocysteine levels, both of which increase clotting risk. These results can change your treatment plan significantly, since an underlying clotting disorder may need lifelong management to prevent further events in your eyes and elsewhere.
Thyroid Eye Disease
Bulging eyes, double vision, eyelid retraction, and a gritty or pressured feeling around the eyes can all signal thyroid eye disease, most commonly linked to Graves’ disease. When your eye doctor suspects this, they’ll order thyroid function tests: TSH, free T4, and T3. In hyperthyroidism, TSH is typically low while T4 and T3 are elevated.
Beyond basic thyroid levels, anti-TSHR antibodies (antibodies that target thyroid-stimulating hormone receptors) are present in virtually all patients with thyroid eye disease. These same antibodies bind to receptors in the tissue around your eyes, triggering the inflammation and swelling that cause symptoms. The levels of these antibodies help predict how severe the eye disease will be and how it’s likely to progress, which makes them useful for planning treatment.
Severe Dry Eye and Sjögren’s Syndrome
Persistent, severe dry eye that doesn’t respond to standard treatments can prompt blood work to check for Sjögren’s syndrome, an autoimmune condition that attacks moisture-producing glands throughout the body. The hallmark blood markers are anti-SSA (also called anti-Ro) and anti-SSB (anti-La) antibodies, which are present in 40 to 80% of people with the condition. Current diagnostic criteria also consider a positive rheumatoid factor combined with an elevated antinuclear antibody level as an alternative marker.
This matters because Sjögren’s syndrome requires a different treatment approach than ordinary dry eye. It also affects other parts of the body, including the mouth, joints, and internal organs, so identifying it through an eye exam can lead to broader medical care you might not have known you needed.
Sudden Vision Loss and Giant Cell Arteritis
This is the most urgent scenario. If you’re over 50 and experience sudden vision loss, especially with a new headache, scalp tenderness, or jaw pain when chewing, your eye doctor will order blood work immediately to check for giant cell arteritis. This condition inflames the arteries supplying blood to the eyes and brain, and without rapid treatment it can cause permanent blindness in one or both eyes.
The critical blood tests are ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein), both of which measure inflammation. An ESR of 50 mm/hour or higher is one of the diagnostic criteria established by the American College of Rheumatology, while a CRP above 20 mg/L raises strong suspicion. Platelet counts above 300 × 10⁹/L add further evidence. These results come back quickly, and if they’re elevated, treatment typically starts the same day, sometimes even before a confirmatory biopsy, because the risk of irreversible vision loss is that high.
What to Expect When Blood Work Is Ordered
In most cases, your eye doctor will send you to a lab with a written order, and results come back within a few days. Some tests, like HLA-B27 or thrombophilia panels, may take longer. Your eye doctor may interpret the results themselves or coordinate with your primary care doctor or a specialist like a rheumatologist or endocrinologist, depending on what the findings suggest.
If your eye doctor orders blood work, it typically means they’ve seen something specific during your exam that points toward a systemic cause. It’s not routine for a standard eye checkup. The results can change your diagnosis, your treatment, and sometimes reveal a condition you didn’t know you had, making the blood draw one of the most valuable things an eye appointment can produce.