Tests are done to:
The normal VIP range is from 0 to 190 pg/ml. A highly sensitive and specific radioimmunoassay is required to detect levels, although this alone is not enough for a diagnosis, as the assay is not totally accurate. A good clinical history is required along with the assay for fasting VIP (secretion may be episodic). As VIP has a half-life of only one minute, special care is required in the sampling. Blood must immediately be added to the protease inhibitor aprotinin, separated within 10 minutes and frozen to minus 20oC. Hypokalaemia, hypercalcaemia, and hypomagnesaemia should also be tested for.
An abdominal X-ray may be of value in showing a small bowel with dilated loops, gall bladder distension, and colon distension.
CT and ultrasound are used to visualise and localise tumours, most of which are solitary, and 8cm or more in diameter. Angiography may also be used for smaller tumours. If VIPoma occurs as part of MEN 1 then transhepatic percutaneous venous sampling may be of value in establishing if a tumour is secreting VIP. 90% of VIPomas are found in the pancreas, the rest are neuroblastomas or gangliomas. VIPomas lying outside the pancreas may be located using somatostatin receptor scintigraphy.