Clinical Biochemistry

Placental alkaline phosphatase (PLAP)

Test Background
Alkaline phosphatases (ALP) are membrane anchored dimeric enzymes. Specific phospholipases can cleave membrane bound ALP and produce free ALP in the serum. ALP consists of a family of four isoenzymes: (1) placental ALP (PLAP) (2) germ cell ALP (PLAP-like), (3) intestinal ALP (IAP), and (4) nonspecific tissue ALP (TNAP, also known as liver/bone/kidney ALP). The PLAP and PLAP-like enzymes share 98 % homology in their amino acid sequence and are both recognised by the PLAP assay.
PLAP is synthesised in the placental syncytiotrophoblast starting after the 12th week of pregnancy. The PLAP-like enzyme is primarily synthesised in the testis, cervix and thymus. In healthy non-smoking adults, PLAP/PLAP-like enzyme activity represents less than 1 % of all serum ALP. Smoking causes elevation of the serum concentrations of the PLAP-like enzyme, limiting the value of PLAP-like enzyme as a tumour marker in smokers. Serum concentrations of PLAP-like enzyme return to the normal range after 1-2 months of smoking cessation.
Ectopic expression of PLAP has been associated with cancer of the ovary, testis, lung and colorectal tract. The highest frequency of serum elevations of PLAP and PLAP-like enzyme are found in patients with ovarian (25 %-65.5 %) and testicular neoplasms 22 %-89 % (20-90 %).

Clinical Indications
Germ cell tumour

• Diagnosis: PLAP may be helpful as an adjunctive test in the clinical work-up of GCT. It is most frequently raised in seminomas.
• Treatment monitoring: in PLAP-positive disease, increasing levels may indicate progression and/or treatment resistance, while declining levels may indicate responsiveness and recovery.
• Surveillance: several studies have shown that if PLAP is used in combination with established markers, a greater sensitivity for relapse detection can be achieved. Specificity is impaired in smokers.

Reference Range

Sample Required
Serum (red top) preferred, SST (gold top) acceptable

Sample Volume
1 mL

Turnaround Time
4 weeks

Notes
Innotest enzyme immunoassay