Physiology: a glucose load is given in order to stimulate insulin secretion – the glucose level in patients with reduced insulin secretion or insulin resistance will fail to return to normal
Preparation
Patient fasted from midnight
Procedure
9am: take glucose level
Give 75g of glucose (e.g. Polycal 122ml) orally within 5 minutes
At 2 hours: take a second glucose level
Interpretation
Diabetes: fasting glucose ≥7mmol/L or 2-hour GTT glucose ≥11.1mmol/L
Physiology: a glucose load is given in order to supress GH production – in patients with acromegaly, GH is secreted inappropriately and will not supress
Preparation
Patient fasted from midnight
Procedure
9am: take glucose + GH level + IGF-1
Give 75g of glucose (e.g. Polycal 122ml) orally within 5 minutes
Every 30 mins: take a second glucose + GH level
Stop at 2 hours
Interpretation
GH should be suppressed to <0.3µg/L in normal people – it is not suppressed in acromegaly
Physiology: a glucose load is given and the patient is monitored for hypoglycaemia – in patients with reactive hypoglycaemia, excess insulin is released after a carbohydrate load which results in hypoglycaemia within 4 hours of eating
Risks: hypoglycaemia
Preparation
Patient fasted from midnight
Procedure
9am: insert cannula (take bloods from this throughout) and take glucose and insulin levels
Check capillary glucose
Give 75g of glucose (e.g. Polycal 122ml) within 5 minutes
Every 30 mins: take glucose, insulin and C-peptide levels
Take additional samples if the patient has hypoglycaemic symptoms and a capillary glucose of <4mmol/L
Check capillary glucose after each sample
Stop at 5 hours and give the patient a snack
Note: blood samples must be taken urgently to lab so the insulin samples can be frozen
Interpretation
Glucose ≤3.5 mmol/L is abnormal and indicates reactive hypoglycaemia
Note: patients with reactive hypoglycaemia almost always spontaneously recover; patients with fasting hypoglycaemia do not recover until they have ingested carbohydrate
72 hour fast
Assesses for: cause of suspected fasting hypoglycaemia
Physiology: patient is subjected to prolonged fast which aims to trigger hypoglycaemia – when this occurs, blood is taken for insulin and C-peptide to determine the cause of hypoglycaemia
At start of fast, take blood for glucose, insulin and C-peptide
Patient is only allowed caffeine-free and calorie-free drinks and their regular medications
Perform capillary glucose measurements 6-hourly until <3.3mmol/L, then 2-hourly
If ever the capillary glucose is <2.5mmol/L or patient is symptomatic, take blood for glucose, insulin and C-peptide
Actions
If the lab glucose is ever confirmed to be <2.5mmol/L with symptoms (or <2mmol/L without) or the patient gets to 72 hours, terminate the test
At the end of the test take blood for beta-hydroxybutyrate (ketones should be supressed in insulinoma due to excess insulin) and sulfonylurea screen, and give patient a sugary drink and some long acting carbohydrate – then monitor capillary glucose every 10mins until >4mmol/L, then 2-hourly until 6 hours
If the patient requires emergency treatment for hypoglycaemia, it should not be withheld
Note: patient is not allowed to leave the ward unaccompanied due to risk of hypoglycaemia and exogenous insulin administration
Interpretation
Venous glucose <2.5mmol/L with symptoms (or <2mmol/L without) is significant
If insulin is high (>3mU/L) and C-peptide is high (>200pmol/L), this suggests insulinoma
If insulin is high (>3mU/L) and C-peptide is low (<75pmol/L), this suggests exogenous insulin administration
If insulin is low (<3mU/L) and C-peptide is low (<75pmol/L), this is appropriate – seek alternative causes of hypoglycaemia (e.g. alcohol, adrenal insufficiency, hypopituitarism, liver failure, mesenchymal tumour etc.)
Water deprivation test
Assesses for: diabetes insipidus
Physiology: patient is subjected to dehydration which normally stimulates the posterior pituitary to release ADH resulting in increased water retention in the kidneys and therefore concentrated urine
Risks: severe dehydration
Preparation
Patient nil by mouth from midnight
If on DDAVP, stop it the evening before (resume after test)
If on any anterior pituitary hormone replacement, these should be taken 1 hour before test
Procedure
Start at 9am
Cannulate (take bloods from this throughout)
At baseline (0 hours) and then every hour check:
Weight, BP and pulse
Urine output (they should hold urine until the hour is up) – weigh the urine in a bowl and then subtract the weight of the bowel
Send urine sample for urine osmolality
Take blood for serum osmolality and U&Es
At 2pm, review results:
If normal (i.e. urine osmolality > 750nmol/kg), stop test
If abnormal, give 2mcg desmopressin IM or IV and continue test
If urine osmolality >750nmol/kg, stop test
Note: if patient looses >3% of bodyweight or serum osmolality is >305 or Na+>150, consider whether to stop test or give fluids and 2mcg desmopressin IM or IV (then continue with free fluids for 2 more hours)
Interpretation
Hypertonic urine (>750nmol/kg) should occur in response to water deprivation and this indicates normality
Hypotonic urine (<750nmol/kg) by 2pm despite water deprivation is abnormal
If the urine then becomes hypertonic (>750nmol/kg) after desmopressin, this indicates cranial diabetes insipidus as the kidneys still respond to ADH
If the urine remains hypotonic (<750nmol/kg) after desmopressin, this indicates nephrogenic diabetes insipidus because the kidneys are resistant to ADH
Sub-maximum urine concentration (500 – 700 mmol/kg) with no response to desmopressin suggests compulsive drinking or partial nephrogenic DI
Insulin stress test
Assesses for: GH and/or cortisol deficiency (in patients with pituitary/hypothalamic disease)
Physiology: insulin is used to induce extreme hypoglycaemia, during which GH and ACTH should be released as part of the stress mechanism
If on oral contraceptive pill, stop this for 6 weeks pre-test (resume after test)
If on hydrocortisone, stop it the evening before (resume after test)
Patient fasted from midnight
Procedure
Ensure medication is available if complications: glucose 20% infusion (10-20ml if patient becomes unconscious); glucagon 1mg IM; hydrocortisone 100mg injection (if patient appears hypoadrenal i.e. hypotensive and tachycardic)
Patient should lie supine
9am: insert cannula (take bloods from this throughout)
At 10mins: take basal cortisol, GH and glucose levels
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