Blood tests are an important tool in detecting and managing cancer, but they rarely offer a definitive diagnosis on their own. The use of bloodwork in oncology ranges from finding indirect clues about a person’s general health to employing sophisticated technology that analyzes the genetic material of tumors. While a single blood test result cannot confirm a cancer diagnosis, it provides data that guides doctors toward the next steps in evaluation. These tests act as a non-invasive surveillance system, offering insights into potential issues and monitoring the body’s response to disease or treatment.
Routine Blood Tests and Indirect Indicators
Standard blood tests, often part of an annual physical, can signal an underlying problem that may be caused by cancer. The Complete Blood Count (CBC) measures red blood cells, white blood cells, and platelets. Abnormalities in these counts can be indirect flags for a potential malignancy, especially blood cancers like leukemia. For instance, unexplained anemia (a low red blood cell count) may occur if a tumor causes chronic bleeding or has spread to the bone marrow, disrupting normal blood cell production.
Changes in white blood cell or platelet levels also prompt further investigation. A very high or very low count of white blood cells could indicate a blood disorder, such as leukemia or lymphoma. Additionally, a comprehensive metabolic panel (CMP) assesses liver and kidney function. Elevated enzymes or high calcium levels can suggest that cancer is affecting these organs or causing metabolic disturbances. These routine tests are not specific for cancer and can be altered by many non-cancerous conditions, meaning they signal a general problem rather than confirming a tumor.
Specific Tests: Tumor Markers
Tumor marker tests look for specific substances, typically proteins, produced either by cancer cells or by the body in response to the cancer. These markers offer additional information when cancer is suspected. Common examples include Prostate-Specific Antigen (PSA) for prostate cancer or Cancer Antigen 125 (CA-125) for ovarian cancer. PSA is a protein made by the prostate gland, and elevated levels can suggest prostate cancer, though they can also be high due to non-cancerous conditions like benign prostatic hyperplasia (BPH) or infection.
The primary function of tumor markers is often to monitor a patient who has already been diagnosed, checking for treatment effectiveness or recurrence. If the marker level falls during therapy, it suggests the treatment is working, while a rise may signal disease progression. A limitation is that they lack the specificity and sensitivity for widespread screening in the general population. They can result in false positives, where a non-cancerous condition causes an elevation, or false negatives, where a cancer is present but does not produce enough of the marker to be detected.
Advanced Detection: Liquid Biopsies
The liquid biopsy analyzes components shed by tumors into the bloodstream. This technology focuses on identifying circulating tumor DNA (ctDNA), which are fragments of genetic material released when cancer cells die. Unlike traditional tumor markers that look for proteins, liquid biopsies search for the DNA mutations characteristic of the cancer itself. This allows for a genetic profile of the tumor from a blood draw.
Liquid biopsies are less invasive than a traditional tissue biopsy and can provide information about tumors that are difficult to access. Current applications include monitoring for minimal residual disease (MRD), detecting tiny amounts of tumor DNA that remain after treatment and may indicate a high risk of relapse. This approach can provide an early warning of recurrence months or years before a new tumor is visible on imaging scans. While promising for screening, the technology is still being optimized for early-stage cancer detection, as not all tumors shed sufficient ctDNA into the blood.
The Role of Blood Tests in Diagnosis and Monitoring
Blood tests provide non-invasive information, but they seldom replace the final steps of diagnosis. For most solid tumors, a definitive diagnosis requires a tissue biopsy, where a sample of the suspected mass is examined under a microscope. Imaging studies, such as CT or MRI scans, are also used to locate the tumor and assess its size and spread. Bloodwork serves to complement these methods by providing biological context and functional data.
In the context of treatment, blood tests are valuable for monitoring the disease’s behavior. Tracking tumor marker levels or ctDNA helps physicians determine if the cancer is shrinking, stable, or progressing, allowing for timely adjustments to therapy. They are also used for post-treatment surveillance, acting as an early warning system for a potential relapse. Blood tests connect the initial suspicion of cancer through to long-term follow-up care.