For 20 yrs my alp is very high in every blood test. latest is 716 u/l. what other symptoms should i look for to narrow down the cause?

ALP: Does this stand for alkaline phosphatase? This can come from bone or liver and there is a blood test that can differentiate the two. If it's the liver then we would want an ultrasound to look at the gall bladder and bile ducts. If it's from bone then you need a workup for bone disease which could be from many different problems and might require xrays or a bone scan and more lab testing to tell.

Alk Phos: High alp levels may suggest bile ducts are obstructed, there could be active bone formation occurring from osteoblast activity (paget's disease). Levels may be elevated in untreated coeliac disease. If you have had this for 20+ years, imagine that this has been assessed by several physicians. If not, find a good internist and investigate. Best wishes.