We constantly encounter reports of predictive blood testing in various diseases. Predictive blood tests can be quite reliable for certain diseases, but have limitations as well. How reliable are such tests, and in case of which diseases, Dr Dipanjan Panda, senior consultant, medical oncology, Indraprastha Apollo Hospital, New Delhi, says, “Tests like the complete blood count (CBC) and basic metabolic panel can accurately screen for anaemia, infection, kidney dysfunction, electrolyte abnormalities and other common conditions. More specialised blood tests reliably predict risk for heart disease, such as cholesterol levels and inflammatory markers like C-reactive protein. Cancer screening blood tests like the PSA test for prostate cancer have moderate reliability—while elevated levels raise suspicion for cancer, they don’t definitively diagnose it.”
Predictive genetic blood tests are emerging for conditions like Alzheimer’s disease. While these can identify high-risk gene variants, most complex diseases are influenced by multiple genes plus lifestyle factors. So while a genetic test might show an elevated risk for eventually developing Alzheimer’s, it cannot conclusively predict if or when someone will be diagnosed.
People diagnosed with late-stage cancer also at times have clean cancer marker reports. How is that possible, and what does it say about such tests?
It is possible for people with late-stage cancer to have clean cancer marker reports for a few reasons:
- No cancer marker test is 100% sensitive. The tests can sometimes produce false negative results, missing cancer cells that are present. The rate of false negatives varies depending on the type of cancer and test used.
- Certain types of cancer do not reliably produce markers that tests detect. If a cancer does not trigger high levels of commonly tested markers like PSA or CA-125, it may evade detection on these blood tests while spreading in the body.
- Cancer cells can undergo genetic changes over time, sometimes ceasing production of previously elevated markers. Cancer is genetically adaptable, finding ways to avoid detection.
A clean cancer marker report in a patient with late-stage cancer highlights the screening limitations. While markers can indicate cancer risk and prompt further testing, they cannot rule out cancer entirely when clinical suspicion is high.
Should people go in for general screening through such tests or only if something specific is suspected?
Opinions differ on broad-based screening versus limited, symptom-driven testing. In favour of broad screening is the argument that many life-threatening diseases like cancer are more treatable when caught early. Universal or wide age-bracket screening programmes for cancers including breast, cervical, colorectal and prostate aim to detect illness before symptoms arise. However, critics argue that overtesting exposes many healthy individuals to unnecessary anxiety, radiation exposure from scans, biopsy complications and other side effects. It can also lead to excessive treatments for non-lethal diseases picked up on screening.
There are merits to both targeted and general screening approaches. The best practice may be disease-dependent, factoring in how critical early detection is for treatment success.
Are there any predictive tests one should have on an annual basis on their own?
While guidelines vary slightly, there are a few common annual screening tests most health providers advise for preventative care. Annual wellness visits should include bloodwork like a basic metabolic panel, complete blood count, and lipid panel to assess organ function, disease risk factors, and baseline health measures. Annual screenings starting at age 45-50 are also suggested for colorectal cancer via home stool tests or colonoscopies every 5-10 years based on risk level. Those sexually active should be tested for STIs as well. Targeted cancer screenings like mammograms for breast cancer and Pap tests for cervical cancer have shorter 2-3 year intervals for those in designated age groups. Finally, discussing family history and reviewing cancer risks help determine if additional targeted genetic testing could provide useful predictive information.