What is PLAP (PALP)?
PLAP, also known as PALP (Placental Alkaline Phosphatase), is an enzymatic protein that belongs to the alkaline phosphatase family. PLAP is normally expressed in the placenta during pregnancy, where it plays a role in phosphate metabolism and transport processes between mother and fetus. In adult men and non-pregnant women, the expression of PLAP is normally very low or absent.
However, in some malignancies, ectopic expression of PLAP may occur. This is seen primarily in germ cell tumors of the testicles, especially seminomas. Elevated levels of PLAP in serum can therefore serve as a biomarker for tumor activity in this context.
Why is PLAP analyzed?
Analysis of PLAP is used in the investigation, diagnosis and follow-up of germ cell cancer, mainly seminomas. The marker can help to strengthen the suspicion of seminoma and is also used to follow disease activity and treatment response over time.
PLAP is not a cancer-specific test and is not used as a stand-alone diagnostic tool. The result should always be interpreted together with clinical symptoms, imaging and other laboratory tests, such as AFP and β-hCG.
What can high levels of PLAP indicate?
Elevated levels of PLAP may indicate ectopic production of the enzyme and are seen primarily in:
- Seminoma
- Other germ cell tumors (less common)
PLAP can also be elevated without malignancy being present. The most important known non-malignant cause is smoking, which can lead to greatly elevated serum concentrations.
What can normal or low levels of PLAP indicate?
Normal or low levels of PLAP generally speak against active PLAP-producing tumor disease. However, a normal value does not completely rule out germ cell cancer, especially not in early stages or in tumors that do not express the marker to a high degree.
Factors that can affect the PLAP value
Several factors can affect the level of PLAP in serum and should be considered when interpreting:
- Smoking (can give values up to approximately tenfold elevated)
- Individual biological variation
- Method-dependent differences between laboratories
Due to the strong association with smoking, PLAP has limited diagnostic value in smokers and should be interpreted with great caution in this group.
Interpretation and clinical use
PLAP is a useful biomarker in the suspicion of or follow-up of seminoma, but is not intended as a stand-alone screening test in asymptomatic individuals. The marker is primarily used as a complement to other tumor markers, clinical examination and imaging diagnostics.
In the event of deviant PLAP values, supplementary investigation with additional tumor markers, radiological diagnostics and clinical follow-up may be necessary to establish the diagnosis and assess the treatment strategy.
PLAP in relation to other germ cell tumor markers
In the investigation of testicular cancer, PLAP is often analyzed together with AFP (alpha-fetoprotein) and β-hCG (human chorionic gonadotropin). These markers reflect different biological properties of germ cell tumors and are used together to distinguish tumor subtypes and follow the course of the disease.