Phosphatases are enzymes that cleave phosphate from compounds with a single phosphate group. Those that are optimally active at pH 9 are grouped under the name alkaline phosphatase (ALP).

ALP is elaborated by a number of tissues. Liver, bone, and intestine are the major isoenzyme sources. During pregnancy, the placenta also is an abundant source of ALP, and certain cancers elaborate small amounts of a distinctive form of ALP called the Regan enzyme. Additional sources of ALP are the proximal tubules of the kidneys, the lactating mammary glands, and the granulocytes of circulating blood (see Chapter 1 - Hematology and Tests of Hematopoietic Function section titled "Leukocyte Alkaline Phosphatase").

Bone ALP predominates in normal serum, along with a modest amount of hepatic isoenzyme, which is believed to derive largely from the epithelium of the intrahepatic biliary ducts rather than from the hepatocytes themselves. Levels of intestinal ALP vary; most people have relatively little, but isolated elevations of this enzyme have been observed. Intestinal ALP enters the blood very briefly while fats are being digested and absorbed, but intestinal disease rarely affects serum ALP levels.

Conditions associated with elevated serum ALP levels, and the magnitude of those elevations, are listed in Table 5-18.²⁸ Numerous drugs also may elevate serum ALP levels.

Decreased levels are seen in cretinism, secondary growth retardation, scurvy, achondroplasia, and, rarely, hypophosphatasia.

• Numerous drugs, including IV albumin, may falsely elevate levels.
• Clofibrate, azathioprine (Imuran), and fluorides may falsely decrease levels.

• Signs and symptoms of disorders associated with elevated ALP levels (e.g., biliary obstruction, hepatobiliary disease, bone disease including malignant processes) (see also Table 5-18)
• Differentiation of obstructive biliary disorders from hepatocellular disease, with greater elevations of ALP seen in obstructive biliary disorders
• Known renal disease to determine effects on bone metabolism
• Signs of growth retardation in children

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving collection of a peripheral blood sample (see Appendix I).

• Because many drugs may alter ALP levels, a medication history should be obtained.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Abnormal levels: Note and report increased levels. Correlate with serum calcium and phosphorus, serum bilirubin, and isoenzymes to determine reason for treatments, progress, and prognosis in diseases of the bone or liver. Assess for jaundice and pathological fracture. If client is pregnant, handle extremities carefully and protect from trauma. Administer ordered vitamin D. Provide comfort measures (soothing bath for pruritus, pain control, support for body image changes) to treat jaundice, if it is present. Advise client to restrict dietary fat.