Fasting and Post-Meal Sugar Readings: What the Numbers Mean

Units and timing

In India, glucose is usually shown in mg/dL. Many other countries use mmol/L. To convert, divide mg/dL by 18 to get mmol/L; for example, 126 mg/dL is about 7.0 mmol/L. Always note the unit in the log.

Timing matters:

• Fasting means no food or drink with calories for at least eight hours, usually the first morning reading before breakfast. Plain water is fine.
• Pre-meal is just before eating.
• Post-meal is usually two hours after the first bite of a meal. Some doctors use one hour.
• Bedtime is just before sleep.
• Random is any time, regardless of meals.

Commonly published ranges for adults

Different bodies publish similar figures. As a general picture for adults, the following are often quoted:

Without diabetes

• Fasting: roughly 70 to 99 mg/dL.
• Two hours after a meal: usually below about 140 mg/dL.

Prediabetes (diagnosed by a doctor using lab tests)

• Fasting: about 100 to 125 mg/dL.
• Two hours after a glucose challenge test: about 140 to 199 mg/dL.

Diabetes (diagnosed by a doctor using lab tests)

• Fasting: 126 mg/dL or higher on repeated lab tests.
• Two hours after a glucose challenge test or a random reading with symptoms: 200 mg/dL or higher.

A home meter reading alone does not make a diagnosis. Diagnosis rests on laboratory tests ordered by a doctor.

Typical targets for people with diabetes

Guidelines such as the American Diabetes Association's Standards of Care and the NICE guideline NG28 suggest that many adults with diabetes aim for:

• Fasting or pre-meal readings in the range of roughly 80 to 130 mg/dL.
• Readings about two hours after a meal under roughly 180 mg/dL.

These are not the same for everyone. Targets depend on age, other illnesses, the medicines used, the risk of hypoglycaemia and a person's wishes.

Why targets are often relaxed in older adults

Healthy older adults with few illnesses may share targets similar to younger adults. But for those who are frail, have several chronic conditions, memory problems or a history of low sugar, doctors often choose more relaxed targets. Published guidance for older adults tends to suggest higher fasting and bedtime ranges, such as fasting readings somewhere around 90 to 150 mg/dL and bedtime readings around 100 to 180 mg/dL for people with complex health. The reason is safety. Very low sugar can cause falls, confusion, heart rhythm problems and even loss of consciousness, and the older body is slower to recognise and correct it. See low blood sugar in older adults.

Ask the doctor: "What range should we aim for before breakfast, after meals and at bedtime for my parent?" Write the answer on the first page of the log.

The HbA1c test

The HbA1c test, done in a lab, reflects the average blood sugar over about the last two to three months. It is usually reported as a percentage. A common cut-off for diagnosing diabetes is 6.5 per cent or higher, and prediabetes is about 5.7 to 6.4 per cent. For people already treated, targets around 7 to 7.5 per cent are often discussed for healthier older adults, and higher, such as 8 to 8.5 per cent, for frail adults or those with limited life expectancy, but the doctor decides. HbA1c can be less reliable in people with anaemia, kidney disease or after blood loss or transfusion.

HbA1c does not show the ups and downs through the day. Home readings fill in that picture: a good HbA1c can hide frequent lows and highs that average out.

Reading patterns, not single numbers

A single reading can be off for many reasons: a sugary fingertip, a very large meal, illness, stress or a poor technique. Look at the pattern over several days:

• High fasting readings may reflect liver sugar release overnight, a late snack or an evening medicine effect. The doctor can look at timing.
• High post-meal readings point to the meal's size and carbohydrate content, such as white rice, rotis, sweets, fruit juice or potatoes.
• High bedtime readings may follow a late meal.
• Low readings in the early morning, mid-morning or afternoon could be tied to missed meals, extra activity or medicines. Tell the doctor.
• Wide swings over the same day deserve a conversation with the doctor.

Bring the log, which should include notes about meals and activity; see how to test blood sugar at home step by step and keeping a health record at home.

Things that change readings

• Food: the type, amount and timing of carbohydrates. Fibre-rich foods and protein slow the rise.
• Medicines: diabetes tablets, insulin, steroids, some blood pressure medicines and others can raise or lower sugar.
• Illness and infection: raise sugar.
• Stress and pain: raise sugar.
• Exercise: often lowers sugar, sometimes for hours afterwards.
• Dehydration: concentrates sugar in the blood.
• Poor sleep.
• Alcohol: can cause low sugar, particularly on an empty stomach.
• Technique: dirty fingers, old strips, heat and humidity.

See glucose strips and lancets guide for avoiding strip problems.

Indian diet and sugar readings

Many Indian meals are rich in refined carbohydrates: rice, rotis, parathas, idli, dosa, poha, and sweets and tea with sugar. Some families find readings jump after festival meals, a wedding or an extra serving of rice. A dietician can help plan portions, timing and food swaps suited to the family's cooking. Do not stop eating or fast drastically to improve readings without the doctor's advice, because that can cause low sugar in people on certain medicines. See diabetes after 60: a home care guide for broader advice.

When high readings need action

Very high sugar can be dangerous. Seek urgent medical help if your parent has:

• A reading that is very high, such as above about 300 mg/dL, with vomiting, abdominal pain, extreme thirst, drowsiness, rapid breathing or confusion.
• A high reading with infection, such as fever, a foot wound or a chest infection.
• A reading above the level the doctor told you to call about.

If your parent is unwell and the reading is high, call the doctor the same day. Do not give extra doses of medicine on your own.

When low readings need action

A reading below about 70 mg/dL, or symptoms such as shakiness, sweating, hunger, confusion, irritability or drowsiness, may mean low blood sugar. Follow the plan the doctor has given. A general approach often taught is to give about 15 grams of fast-acting sugar, such as glucose tablets or half a glass of fruit juice, then recheck in 15 minutes, but follow your own doctor's plan and see diabetes and low-sugar emergencies. If your parent is unconscious, cannot swallow or is having a seizure, do not put food or drink in the mouth; call 112.

Do home readings match the lab?

Not exactly. Home meters use capillary blood from the finger, and labs use venous plasma. Differences of up to roughly 15 per cent are common. Compare your meter with the lab at the doctor's request, and use the same meter consistently. A reading that does not match how your parent feels should be repeated.

Avoiding obsession

Testing many times a day can cause anxiety, especially in an older person who fears a "bad number". The log should serve the doctor's plan, not become a source of guilt. Praise efforts, discuss patterns with the doctor and keep testing to the schedule the doctor set.

When to ask a professional

Show the log to the doctor or diabetes nurse and ask what ranges apply, how often to test, and what to do at which numbers. Ask a dietician for meal planning. Ask for help if readings are regularly outside target, if there are repeated lows, if your parent has symptoms despite normal readings, or if the meter and lab disagree. Call 112 for unresponsiveness, seizure, severe drowsiness or confusion, or breathing difficulty.