Can a blood test Miss diabetes?

Diabetes is a chronic health condition that affects how the body turns food into energy. With diabetes, the body either doesn’t make enough insulin or can’t effectively use the insulin it produces, leading to high blood sugar levels. Diabetes is a serious disease that can lead to various complications if left unmanaged, including nerve damage, kidney disease, vision loss, stroke, and heart disease. For these reasons, getting an accurate diabetes diagnosis through proper testing is extremely important.

What tests are used to diagnose diabetes?

There are several tests that can be used to diagnose diabetes:

  • A1C test: This blood test measures your average blood sugar over the past 2 to 3 months. An A1C of 6.5% or higher indicates diabetes.
  • Fasting plasma glucose (FPG) test: This test measures your blood sugar after fasting for at least 8 hours. A fasting blood sugar level of 126 mg/dL or higher suggests diabetes.
  • Oral glucose tolerance test (OGTT): For this test, you fast overnight and then drink a sugary liquid. Your blood sugar is measured before and 2 hours after drinking the solution. Blood sugar levels meeting the threshold at 2 time points during the test indicate diabetes.
  • Random blood sugar test: This test measures blood sugar at any time. A random blood sugar level of 200 mg/dL or higher combined with symptoms of diabetes could indicate diabetes.

Out of all these tests, the A1C, FPG, and OGTT are the primary tests used to officially diagnose diabetes. The random blood sugar test alone is not sufficient, but may prompt further testing if results are very high.

Can blood tests miss a diabetes diagnosis?

Yes, it is possible for the common blood tests used to diagnose diabetes to miss the diagnosis. Here are some reasons how and why this can happen:

  • Testing was done too early: If you are tested very early on when diabetes symptoms first appear, blood sugar levels may not yet be high enough to meet the thresholds for diagnosis. Repeating the test after some more time passes often catches cases that were initially missed.
  • Normal test results despite classic symptoms: Sometimes individuals present with classic signs and symptoms of diabetes like frequent urination, increased thirst, unexplained weight loss, fatigue, etc. but their test results don’t reflect diabetes at that moment in time. This may be because blood sugar fluctuates throughout the day. Symptoms combined with risk factors like family history may warrant retesting or additional testing methods.
  • Testing during an illness or infection: Illness or infection can temporarily spike blood sugar levels. This could lead to a false positive result for diabetes when the actual issue resolves after the illness. Retesting after recovery is recommended.
  • Normal results in early stage or type 1 diabetes: Early on in type 1 diabetes, blood sugar levels may remain normal because the body is still producing some insulin. Over time, insulin production drops off leading to abnormal test results. So early diabetes may be missed but become detectable later on.
  • Testing after significant diet changes: Major changes in diet and eating patterns can impact test results. Abnormal findings may reflect recent dietary changes rather than true diabetes. Retesting after some time on a typical diet helps rule out this factor.
  • Improper test administration/procedure: Errors in how the test was performed, such as not fasting properly prior for a fasting blood glucose test, can lead to incorrect results and missed diagnosis.

For these reasons, it is important to remember that blood tests for diabetes are not perfect and the diagnosis should not be ruled out based on a single normal result.

How often should testing be repeated?

If diabetes is suspected based on risk factors or symptoms but initial blood test results are normal, the American Diabetes Association recommends repeat testing at least annually. Higher risk individuals may need testing more frequently:

  • Every 6-12 months for adults with prediabetes (A1C of 5.7-6.4%)
  • Every 3 years for adults age 45+ with normal results but risk factors present
  • Every 1-2 years if you are overweight and have additional risk factors
  • Every 1-3 years if you have no risk factors but are overweight or obese

Doctors may recommend retesting much sooner, such as in 3-6 months, if diabetes is strongly suspected clinically but not confirmed with the first test. Annual testing is recommended to recheck negative results and monitor those at higher risk.

What are other diagnostic options if blood tests are normal?

If blood test results do not explain symptoms that suggest diabetes, here are some other diagnostic options a doctor may consider:

  • Hemoglobin A1C test: Since this test reflects average blood sugar over 2-3 months, it may catch high blood sugars that random testing misses.
  • 2-hour oral glucose tolerance test: This test requiring fasting and then drinking a sugary drink may reveal issues with processing sugars that a fasting-only test misses.
  • Serial blood glucose testing: Testing blood sugar periodically throughout the day can help pinpoint abnormal spikes after meals that fasting tests miss.
  • Checking urine ketones: Persistently high ketones in the urine suggest insulin deficiency and undiagnosed diabetes.
  • Insulin level testing: Directly measuring insulin levels can reveal insulin deficiency signaling diabetes, especially in early type 1 disease.
  • Genetic testing: Testing for gene variants linked to diabetes risk may reveal a genetic predisposition requiring closer monitoring.
  • Clinical trial participation: Research studies related to diabetes often perform intensive diagnostic testing that could lead to a diagnosis.

Doctors also may diagnose diabetes or prediabetes based on your reported symptoms, risk factors, and physical exam findings even if blood test criteria are not (yet) met. Ongoing monitoring would be required in such cases.

What are warning signs and symptoms not to ignore?

Certain signs and symptoms could indicate diabetes even if initial blood tests are normal. Symptoms not to ignore include:

  • Frequent or increased urination
  • Excessive thirst
  • Extreme hunger
  • Unexplained weight loss
  • Blurry vision
  • Slow healing cuts/bruises
  • Tingling, pain, or numbness in hands/feet
  • Feeling very tired or fatigued
  • Very dry skin
  • Frequent infections

Pay attention to these red flags and persist advocating for yourself with your doctor if blood tests are not lining up with concerning symptoms you are experiencing. A second opinion from an endocrinologist may also be worthwhile.

Who is at higher risk of a missed diagnosis?

There are certain groups that seem to be at higher risk of having diabetes missed on initial testing. Those more likely to have false negative blood test results include:

  • Children and adolescents: Type 1 diabetes is on the rise among youth but may not be initially suspected. Early signs can be vague and blood sugars may not be severely elevated yet.
  • Pregnant women: Gestational diabetes testing sometimes misses cases, and the hormonal changes of pregnancy can obscure other types of diabetes.
  • Thin individuals: Type 2 diabetes is less suspected in normal or underweight individuals, leading to missed diagnoses.
  • Ethnic minorities: False negatives appear to be more common among certain ethnicities, like African Americans, Hispanics, and Asians.
  • Older adults: Hard-to-spot symptoms, lack of routine testing, and complications from age-related conditions contribute to missed diagnoses in seniors.

Doctors need a high index of suspicion for diabetes in these groups even if initial bloodwork is reassuring. Ongoing screening and retesting is important when symptoms arise.

What role do risk factors play in missed diagnoses?

Diabetes risk factors that increase someone’s chances of developing diabetes also increase the likelihood of a missed diagnosis. Doctors may falsely assume low risk and not test rigorously enough when risk factors are actually present. Key risk factors include:

  • Being overweight or obese
  • Older age (45+ years old)
  • Family history of diabetes
  • History of gestational diabetes or giving birth to a large baby
  • Sedentary lifestyle
  • Race/ethnicity (e.g. African American, Hispanic American, Native American, Asian American, Pacific Islander)
  • High blood pressure
  • Abnormal cholesterol levels
  • Polycystic ovarian syndrome (PCOS)
  • Prediabetes diagnosis

Doctors should test for diabetes if any of these risk factors are present, even in the absence of symptoms. Risk factors combined with concerning symptoms should warrant an intensive investigation including potential retesting.

How can these missed diagnoses be prevented?

Here are some tips to help prevent missed or delayed diagnoses of diabetes:

  • Get tested if you have any risk factors, even without symptoms.
  • Insist on retesting if symptoms develop after a normal test.
  • Make sure proper procedure is followed when getting tested.
  • Request the optimal tests (A1C, fasting glucose, glucose tolerance) rather than just a random check.
  • Ask your doctor to check insulin and c-peptide levels if diabetes is suspected based on symptoms.
  • Keep track of your numbers if you are at higher risk and monitor for trends.
  • Follow up consistently if initial results are borderline.
  • See an endocrinologist if your primary doctor is not investigating worrisome symptoms.

Do not let one normal blood test result rule out diabetes if you have classic signs of the disease. Be proactive with retesting and additional diagnostic options. Catching diabetes early is critical to prevent complications.

What complications can occur from missed diagnosis?

When diabetes goes undiagnosed for too long, severe and potentially life-threatening complications can develop. Consequences from delayed diagnosis include:

  • Diabetic retinopathy (eye disease): Blurry vision can turn into permanent vision loss and blindness.
  • Diabetic neuropathy (nerve damage): Tingling and numbness leads to severe nerve pain, weakness, and reduced mobility.
  • Kidney disease (nephropathy): Excess sugar damages blood vessels in the kidneys, leading to irreversible kidney decline/failure.
  • Cardiovascular disease: Increased risk of heart attack, stroke, and blockages in blood vessels, especially arteries.
  • Foot damage: Poor circulation and reduced sensation allows minor foot issues to escalate into infected ulcers, gangrene, and need for amputation.
  • Skin conditions: Bacterial and fungal infections become more common.
  • Hearing impairment: Higher risk of hearing loss and ear issues.
  • Alzheimer’s disease: Diabetes raises risk of dementia, including Alzheimer’s.

In the worst cases, undiagnosed diabetes can even lead to death from diabetic coma or ketoacidosis caused by extremely high blood sugars. Getting accurate testing when diabetes is first suspected, rather than dismissing early symptoms, is key to avoiding these devastating complications of missed or late diagnoses.

Should doctors rely less on blood tests alone?

Blood tests for diabetes provide an objective measurement tool, but they have limitations. Doctors probably should not solely rely on blood test results alone if a patient’s symptoms and risk factor profile suggests diabetes as a likely possibility. Some ways doctors could improve diagnosis include:

  • Putting more emphasis on diabetes symptoms when deciding who to test.
  • Testing at-risk patients even if they are asymptomatic.
  • Repeating testing in 3-6 months if initial results don’t match the clinical picture.
  • Using additional diagnostic tools like insulin/c-peptide levels and ketone tests.
  • Referring complex cases to endocrinologists sooner.
  • Encouraging patients to monitor blood sugars at home if concerned.
  • Starting preventative interventions like diet changes with prediabetes.

Doctors also need to stay up-to-date on the latest diabetes screening recommendations and diagnostic criteria, which change over time as new evidence emerges. Overall, using blood test results as just one component of a comprehensive diagnostic evaluation, rather than as the final word, can improve early diabetes detection and save lives.


Blood tests like the A1C and fasting glucose are powerful tools, but they are not perfect when it comes to detecting diabetes in all individuals. Missed diagnoses absolutely can and do occur. Being aware of the limitations and nuances of diabetes testing allows patients and doctors together to find cases that might slip through the cracks with routine screening alone.

Knowing the scenarios where false negatives are more common, and combining testing with a thorough symptom and risk factor review, reduces missed diagnoses. Patients also need to be persistent advocates for themselves if diabetes is suspected but not confirmed with initial results. Retesting periodically and using other diagnostic tests are key for diagnosis. Early detection of diabetes leads to better health outcomes, so it is important not to dismiss worrisome symptoms if blood tests are normal at first. With proper tracking and follow up, the potentially devastating complications of missed diabetes diagnoses can be reduced or avoided.

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