Serum proteins
Tumour markers are secreted proteins produced by cancers that are detectable in the serum of patients. Some have found clinical application as a means of monitoring the course of disease and as prognostic factors. Tumour markers may be used for population screening, diagnosis, prognostic factors, monitoring treatment, diagnosis of relapse and imaging of metastases. The minimal requirements for a tumour marker are:
- reliable, quick, cheap assay;
- high sensitivity (>50%) and specificity (>95%);
- high predictive value of positive and negative results.
Clinical usefulness
The sensitivity of a test is defined as the percentage of patients with a particular disease who have elevated marker levels and are therefore true positives. Specificity is the percentage of patients without disease who have normal marker levels and are therefore true negatives. The positive predictive value is the percentage of positive results (i.e. elevated marker levels) that are true positives. The false-positive rate is the percentage of patients without disease who have an elevated marker level. The false-negative rate is the percentage of patients with disease who have a normal marker level.
Human chorionic gonadotrophin
Human chorionic gonadotrophin (hCG) is a glycoprotein formed physiologically in the syncytiotrophoblast of the placenta, which is used to diagnose and monitor pregnancy, gestational trophoblastic disease and germ cell tumours. The sensitivity is 100% for testicular and placental choriocarcinomas and hydatidiform moles, 48–86% for NSGCT and 7–14% for seminomas. This is the closest model to a perfect tumour marker.
Alpha-fetoprotein
Alpha-fetoprotein (AFP) is synthesised by fetal yolk sac, liver and intestine, and in the fetus is the major serum protein acting as an albumin-like carrier protein. Moderately elevated levels are seen in some patients with pancreatic, biliary, gastric and bronchial cancers, as well as occasional patients with non-malignant hepatic disease where active hepatic regeneration is occurring. Increased levels are found in patients with hepatocellular carcinoma and GCT of the testes, ovary and midline structures, including mediastinum and pineal gland, that contain yolk sac tissue. Serum values occasionally increase as a result of chemotherapy.
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