Fasting Plasma Glucose 159 mg/dL but 2-Hour Blood Sugar 109 mg/dL: Am I Diabetic?

by K.T.Rao
(Hyderabad, India)



Question from K.T. Rao:


My fasting plasma glucose is 159 mg/dL, while the reference range is 70–100 mg/dL. My 2-hour post-lunch blood sugar is 109 mg/dL, while the reference range is 100–140 mg/dL.


Am I diabetic?




Quick Answer


A fasting plasma glucose of 159 mg/dL is in the diabetes range if the test was done after at least 8 hours of fasting and is confirmed by repeat testing or another accepted test.


Your 2-hour post-lunch reading of 109 mg/dL is reassuring, but it does not cancel the high fasting result.


Please repeat the fasting plasma glucose test and ask your doctor for an HbA1c. If needed, your doctor may also advise an oral glucose tolerance test.



Answer by Dr. Albana Greca, MD, MMedSc


Hello K.T. Rao,


I understand your concern. Your fasting plasma glucose result needs proper follow-up because 159 mg/dL is clearly above the usual fasting range.


Does Fasting Glucose 159 mg/dL Mean Diabetes?


For many nonpregnant adults, fasting plasma glucose is interpreted this way:



  • Below 100 mg/dL: usually normal

  • 100–125 mg/dL: prediabetes range

  • 126 mg/dL or higher: diabetes range if confirmed


So, your fasting result of 159 mg/dL is not borderline. It is high enough that diabetes must be considered and confirmed properly.


Read more here: blood tests for diabetes.


Why Is the 2-Hour Reading Normal?


A 2-hour post-lunch reading of 109 mg/dL is within a good range. However, fasting and after-meal readings can sometimes look different.


Possible reasons include overnight liver glucose release, the dawn phenomenon, insulin resistance mainly in the fasting state, a smaller or lower-carbohydrate lunch, exercise, stress, poor sleep, illness, or testing variation.


You may find this helpful: why blood sugar is high in the morning.


What Should You Do Next?



  • Repeat fasting plasma glucose through a reliable laboratory.

  • Make sure you fast for at least 8 hours, drinking only water.

  • Ask for an HbA1c test.

  • Tell your doctor if you had fever, infection, stress, poor sleep, steroid medicine, or recent diet changes.

  • Ask whether an oral glucose tolerance test is needed if results remain unclear.


Helpful pages: HbA1c explained and blood glucose tolerance test.



When to Contact a Doctor


Please contact your doctor soon because a fasting plasma glucose of 159 mg/dL needs confirmation and interpretation with your age, weight, symptoms, family history, medicines, and other health conditions.


Seek urgent care if you have very high readings with vomiting, severe weakness, confusion, dehydration, deep or rapid breathing, chest pain, severe abdominal pain, or fruity-smelling breath.




Doctor’s Note


Your fasting glucose is high enough to require follow-up, even though your 2-hour post-lunch reading is normal. Do not panic, but do not ignore it. Repeat the fasting test and check HbA1c with your doctor.




Educational safety note: This answer is for general diabetes education only. It does not replace personal medical advice, diagnosis, or treatment. Please review your results with your own doctor before making treatment decisions.



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