Cyfra 21-1 (Cytokeratin 19) is a blood test that measures the level of cytokeratin 19 fragment in the blood. It is a protein released from epithelial cancer cells, especially lung cancer. The Cyfra 21-1 test is used to diagnose, monitor treatment, and predict the prognosis of non-small cell lung cancer. The test can also be used to evaluate treatment effectiveness and detect early recurrence of the disease.