Obstructive jaundice is best detected by:

Alkaline phosphatase is a marker of cholestasis i.e. elevation of ALP occurs in >90% of patients with cholestasis and suggests a reduction in bile flow. Because isozymes are found in the liver, bone, placenta, leukocytes, and small intestine, an elevated ALP is not specific for the biliary tract. ALP levels of greater than four times the normal are taken significant.