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🚫 Fasting Required ⏱ 10 min (FBG/HbA1c) or 2.5 hours (OGTT)

🩺 How to Prepare for a
Diabetes Screening (Blood Sugar Tests)

Diabetes screening uses blood tests to measure how your body handles glucose. A fasting blood glucose (FBG) and HbA1c can be done in one visit with a standard fasting blood draw. The oral glucose tolerance test (OGTT) is a 2–3 hour extended test used to diagnose gestational diabetes and borderline cases. Each has specific preparation requirements.

✅ Preparation Steps

1
Fast for 8–12 hours for FBG and OGTT
If you are having a fasting blood glucose or an OGTT, you must fast completely for 8 hours minimum (10–12 hours is preferred) before the test. Water is allowed freely. No food, juice, tea, coffee, or sweets during the fast. The HbA1c test does NOT require fasting — it can be done at any time of day.
2
Avoid strenuous exercise in the 24 hours before an OGTT
Vigorous exercise significantly lowers blood glucose and can cause false-negative OGTT results. Take the day before and the morning of an OGTT as a rest day. Light walking is acceptable.
3
Do not smoke on the day of the OGTT
Smoking acutely raises blood glucose by stimulating stress hormones. Avoid cigarettes on the morning of your OGTT until all three blood draws are complete.
4
Take regular medications unless specifically told otherwise
Antihypertensives, statins, and most common medications may be taken normally. However, metformin and other diabetes medications may be withheld on OGTT day by your doctor if the test is being used to assess how your body responds to glucose without medication. Confirm with your prescribing doctor in advance.
5
For OGTT — plan to stay at the clinic for 2–3 hours
The OGTT involves three blood draws: one fasting, one at 1 hour, and one at 2 hours after drinking a standardised 75g glucose drink. You must remain at the clinic throughout and cannot eat, drink (except water), or exercise between draws. Bring something to read or do.
6
If you are pregnant and having a gestational diabetes test (GDM screen)
The GDM OGTT is the standard for gestational diabetes diagnosis in pregnancy. Follow the same fasting and exercise guidelines. The glucose drink may cause mild nausea in some pregnant women — sipping slowly helps. Avoid taking the test in the first trimester (it is standardly done at 24–28 weeks).

🎒 What to Bring

🔬 What to Expect

Fasting blood draw (FBG)
A single blood tube is taken from the arm after confirming your fasting status. Takes 2–3 minutes. Result available within 2–4 hours. Fasting glucose ≥7.0 mmol/L on two occasions = diabetes; 6.1–6.9 = impaired fasting glucose (pre-diabetes).
HbA1c blood draw
A single blood tube. No fasting required. Result available within 24 hours. HbA1c ≥48 mmol/mol (6.5%) = diabetes; 42–47 mmol/mol (6.0–6.4%) = pre-diabetes.
OGTT — 75g glucose drink
After the fasting draw, you will be given a 75g glucose solution (dissolved in 250–300 mL of water) to drink within 5 minutes. It is very sweet — some find it nauseating. Sit quietly and do not eat or exercise between draws.
OGTT — 1-hour and 2-hour draws
Blood is drawn at exactly 1 hour and 2 hours after you finish the glucose drink. The 2-hour value is the diagnostic result: ≥11.1 mmol/L = diabetes; 7.8–11.0 mmol/L = impaired glucose tolerance.

🏠 After Your Test

Frequently Asked Questions

What is the difference between FBG, HbA1c, and OGTT?
FBG (fasting blood glucose) measures your blood sugar at a single point in time after fasting — it reflects your current baseline. HbA1c measures average blood sugar over the past 3 months by assessing how much glucose has attached to haemoglobin in red blood cells — it does not require fasting and is unaffected by a single bad meal. OGTT tests how your body handles a large glucose load over 2 hours — it is the most sensitive test for detecting early glucose intolerance, especially in pregnancy. All three are used together in clinical practice.
My FBG was 6.4 — do I have diabetes?
6.4 mmol/L falls in the "impaired fasting glucose" or pre-diabetes range (6.1–6.9 mmol/L). This is not diabetes — but it is a clear warning that your insulin sensitivity is declining. It calls for lifestyle change: reducing refined carbohydrates, increasing physical activity, and losing 5–10% body weight if you are overweight. With these changes, many people with IFG return to normal glucose levels within 6–12 months. Repeat fasting glucose and HbA1c in 3–6 months.
Can I have a normal HbA1c but still have diabetes?
Rarely, yes. HbA1c can be falsely low in: sickle cell anaemia or sickle cell trait (very common in Nigerians), severe iron deficiency anaemia, recent blood transfusion, and haemolytic anaemia. If there is strong clinical suspicion of diabetes but HbA1c is normal, an OGTT or fasting glucose provides an alternative diagnostic answer.
Is gestational diabetes dangerous?
Uncontrolled gestational diabetes (GDM) increases the risks of: large baby (macrosomia), difficult delivery, neonatal hypoglycaemia, and stillbirth. However, GDM diagnosed and managed promptly — with dietary changes and, if necessary, insulin — has excellent outcomes for both mother and baby. The importance of GDM screening at 24–28 weeks is that the majority of cases are completely asymptomatic.

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