Intravenous Secretin Test

INDICATION

The intravenous secretin test should, whenever possible, be performed only after the results of basal plasma gastrin and acid output - both performed off PPIs for 2 weeks, H2 blockers for 3 days and fasted - are available.
The indications for the test are:

1. Strong clinical suspicion of a gastrinoma with equivocal results of acid studies and fasting gastrin.
2. Inability to wean patients off antisecretory therapy for long enough to perform acid studies and gastrin estimation because of recurrence of severe symptoms.

PREPARATION

Warn fasting gut hormone lab 48 hours in advance.
Fast overnight. If possible, stop antisecretory therapy for 24 hours.
Secretin (Kabi) ordered in advance from Pharmacy.
7 x 7 ml Lithium Heparin tubes (green top Vacutainers) with 200µl Trasylol labelled before the study.
Syringes.
Ice.
Arrangements to transfer for immediate spinning.

METHOD

1. Site indwelling cannula.
2. Take two baseline samples at T = -15 and 0 mins.
3. Secretin 2U/kg injected as bolus at T = 0.
4. Blood samples taken at T = 2, 10, 15, 20 and 30 minutes.
5. Samples stored on ice and spun within 15 minutes.
6. All samples assayed for gastrin.

INTERPRETATION

The criteria for diagnosing a gastrinoma are based on gastrin assays from other laboratories where results may not be directly comparable. The best criterion is a rise in gastrin of 200 pg/ml - equivalent to about 100 pmol/l. This gives a sensitivity of 85% when performed on all patients with a fasting gastrin of less than 400 pmol/l. A rise of 50% over basal values gives a sensitivity of 78%. Gastrin levels FALL in normal individuals in response to secretin.
Few false positives have been reported, but massive rises occasionally occur in association with achlorhydria and common duodenal ulcer disease, hence the need to have acid studies and fasting gastrin as the initial investigations, if possible.