Blood test can find early cancer but unfortunately have too high of a false positive rate to get FDA approval. Most tests claim high sensitivity, but the other key aspect of accuracy is specificity (avoiding false positives)
Across all cancers, 20% of patients are told they are cancer free only to have that cancer recur. In some cancers this rate is even higher; 30% of patients with colorectal cancer (CRC) thought to be cancer free have a recurrence. Additionally, 7% of cancer patients suffer debilitating fear of cancer recurrence. The current standard of care for diagnosing cancer recurrence is radiological imaging, which often cannot diagnosis cancer recurrence until it is too late to cure the cancer.