Rank the ALP isoenzymes by heat stability from most to least stable.

• A. P I L B
• B. B L I P
• C. P B I L
• D. I P L B

Answer: (A)

Heat stability testing uses heating to distinguish ALP isoenzymes by whether they retain activity after warming. Placental ALP is the most heat-stable and remains active after heating. Intestinal ALP is next in stability, retaining more activity than the others aside from placental. Liver ALP is more heat-labile than intestinal, so it loses more activity with heating, but not as completely as bone. Bone ALP is the least heat-stable and becomes inactivated most readily with heat.

So the order from most to least heat-stable is placental, then intestinal, then liver, then bone.