While blood tests are valuable tools in cancer care, they generally do not provide a definitive diagnosis on their own. A comprehensive approach involving multiple diagnostic methods is often required to confirm a cancer diagnosis.
Blood Tests Used in Cancer Screening and Detection
General blood tests can offer initial clues suggesting cancer or a need for further investigation. A complete blood count (CBC) measures different blood components, and abnormalities might raise suspicion. For instance, unusually low red blood cell counts (anemia) could be a secondary effect of certain cancers or a direct sign of blood cancers like leukemia or lymphoma. Abnormal white blood cell or platelet counts might also suggest these types of blood cancers.
Blood protein tests also provide insights. High levels of certain proteins, such as globulins, might point towards blood cancers like multiple myeloma. Conversely, low albumin levels may indicate cancer affecting the liver or overall nutritional status. These tests help doctors understand organ function and can reveal changes warranting a closer look.
Liver and kidney function tests assess the health of these organs by measuring various blood substances. Abnormal results might suggest cancer is affecting these organs directly or has spread. While these general blood tests do not diagnose cancer definitively, they can provide important indicators that guide further diagnostic steps.
Understanding Cancer Markers in Blood
Cancer markers (also known as tumor markers or biomarkers) are substances produced by cancer cells or by the body in response to cancer. These markers, often proteins, can be found in higher amounts in the blood, urine, or tissues when cancer is present. Their levels provide information about certain cancers and are used alongside other tests.
Common cancer markers offer specific insights. Prostate-specific antigen (PSA) is primarily used for prostate cancer, with elevated levels indicating its presence. Carcinoembryonic antigen (CEA) can be elevated in various cancers, including colorectal, lung, breast, and ovarian cancers, and is often used to monitor treatment or detect recurrence. CA-125 is frequently associated with ovarian cancer, though its levels can also be elevated by non-cancerous conditions.
Alpha-fetoprotein (AFP) may indicate liver cancer or certain germ cell tumors. Human chorionic gonadotropin (hCG), a hormone associated with pregnancy, can also be a marker for germ cell tumors, including those of the testes and ovaries. Beyond established markers, emerging technologies like circulating tumor DNA (ctDNA) or liquid biopsies detect genetic material released by cancer cells into the bloodstream, offering non-invasive insights into tumor biology.
Why Blood Tests Alone Are Not Enough
Many tumor markers are not specific to a single cancer type, and their levels can be elevated by non-cancerous conditions like inflammation, benign tumors, or infections, leading to false positives. For example, elevated CA-125 levels, while associated with ovarian cancer, can also occur due to endometriosis or liver disease. Similarly, CEA levels can increase due to conditions like peptic ulcers or pancreatitis.
Conversely, a negative blood test result does not always mean cancer is absent. Some cancers may not produce detectable levels of markers, or levels might not be elevated in early stages, resulting in false negatives. This lack of sensitivity, especially in early stages, means blood tests cannot reliably rule out cancer on their own. Therefore, a tissue biopsy (examining a cell sample under a microscope) is required for a conclusive cancer diagnosis.
Blood test results are interpreted within a broader clinical context, alongside physical examinations, patient history, and imaging studies (e.g., X-rays, CT scans, MRIs). This comprehensive approach helps medical professionals avoid misinterpretations based on blood work alone.
Blood Tests Beyond Initial Detection
Once a cancer diagnosis is established, blood tests play several important ongoing roles. They are widely used to monitor the effectiveness of cancer treatment. If tumor marker levels, such as CEA or CA-125, decrease during therapy, it often indicates that the treatment is working and the cancer is responding. Conversely, stable or rising levels might suggest that the treatment is not as effective or that the cancer is progressing.
Blood tests are also crucial for detecting cancer recurrence after treatment has concluded. Regular monitoring of marker levels can identify if cancer has returned, sometimes months before it can be seen on traditional imaging scans. For instance, a persistent rise in CEA after colorectal cancer treatment can signal a recurrence. Newer tests, like those detecting circulating tumor DNA (ctDNA), can identify tiny traces of cancer DNA in the blood, offering highly sensitive detection of minimal residual disease and potential recurrence.
Certain blood tests can provide information about a cancer’s aggressiveness or likely outcome, contributing to prognosis assessment. General blood tests, such as a complete blood count, are also routinely performed to monitor for potential side effects of cancer treatments like chemotherapy or radiation, helping doctors manage patient health throughout their journey.