The glucose tolerance test and Cystic Fibrosis


The oral glucose tolerance test (OGTT) is the definitive test for diabetes mellitus in the general population and is the “gold standard” for diagnosis of diabetes mellitus in patients with CF.

1) All pancreatic insufficient patients over the age of ten years (who do not already have CFRD) should have an annual OGTT.

2) The OGTT should be performed at a time of clinical stability.

3) The OGTT should NOT be performed during acute illness, shortly after major surgery or while the patient is on oral steroids.

4) Where possible factors which may affect or influence the results of the OGTT should be standardised. Variations in the protocol should be kept to a minimum.

NB. There are minor variations between the Paediatric Unit and the Adult Unit protocols.


a) Fast the patient for twelve hours.

b) The test should be performed in the morning.

c) The patient should remain sitting quietly during the OGTT and avoid physical activity.

d) Adequate carbohydrate should be consumed on the days preceding the test.

e) Check drug therapy to determine if any drugs will affect blood glucose concentration (e.g. concomitant corticosteroid therapy).

Administration of glucose load

1) A glucose load equivalent to 75g of anhydrous glucose should be administered in a volume of 300 ml fluid. A partial hydrolysate of starch e.g. Polycal (Nutricia Clinical Care, White Horse Business Park, Trowbridge, Wiltshire, BA14 0XQ) is an acceptable alternative.

2) In children and those patients weighing less than 43 kg the glucose load should be determined individually using 1.75 g anhydrous glucose per kg body weight to a maximum of 75 g glucose.

3) The solution should be at room temperature.

4) The solution should be ingested over five minutes with the timing of the test beginning at the start of ingestion.

Blood sampling

1) Blood samples should be taken at 60 minutes and 120 minutes (Fluoride Oxalate Tube). Ensure that both bottles are labelled with time and date for the labs to analyse the results.

2) We do not currently measure fasting glucose.

Interpretation of results

Based on WHO criteria for diabetes and impaired glucose tolerance

Based on WHO criteria for diabetes and impaired glucose tolerance

Venous sample
120 min sample <7.8 mmol/l Normal
Venous sample 120 min sample ≥7.8 mmol/l and <11.1 Impaired
Venous sample 120 min sample ≥11.1 mmol/l Diabetic

Action/Leeds CF Unit Treatment protocol

Normal glucose tolerance

120 min sample <7.8 mmol/l

Repeat OGTT one year
Impaired glucose tolerance test120 min sample >7.8 mmol/l and <11.1 Repeat OGTT in six months unless indicated
Diabetic glucose tolerance test
a) Blood sugars high
b) Blood sugars normal
Home blood sugar monitoring before and 90 minutes after meals for seven daysTreat with insulin

Repeat OGTT in six months/BM chart at home

Glucose monitoring for patients on overnight feeds

All patients – Blood glucose levels pre and post feed should be measured on three occasions at the beginning of each hospital admission.

If normal – no action.
If in doubt – perform two hourly blood glucose during the night
If high (>11mmol/l) – start night time dose of insulin. In addition monitor 24 hour blood glucose profile (pre-meal and one and a half hours post meal blood sugar levels). If these are high start day-time insulin. Continue measuring blood glucose during the admission before the patient is sent home on insulin.

Glucose monitoring for patients on oral corticosteroid therapy

Oral steroids can affect glucose tolerance. This problem mainly occurs as children get older. Glucose tolerance should therefore be checked in all patients over 10 years of age when starting oral steroids and in younger children when indicated.

1st stage: Urine analysis measurements may be used in children but is less sensitive than blood glucose monitoring. If urinalysis is normal no further action is required.

If high:

2nd stage: Check blood glucose nine hours after steroid dose and perform blood glucose profile for three days (as described above).

Action: As described above. Isophane insulin may be the most appropriate to cover the action of the steroids.


Check blood glucose nine hours after steroid dose and perform blood glucose profile for three days (as described above).

Action: As described above.

As steroid therapy is reduced it is likely that glucose tolerance will revert to normal. It is therefore important that regular blood glucose monitoring is carried out if insulin therapy has been started.