What Illnesses Can Be Detected by Blood Tests?

Blood tests can detect or help diagnose an extraordinarily wide range of conditions, from common infections and nutrient deficiencies to heart disease, diabetes, autoimmune disorders, certain cancers, and even early signs of Alzheimer’s disease. A single tube of blood carries information about organ function, immune activity, hormone levels, genetic material, and traces of drugs or toxins. The scope is broad enough that most people will encounter dozens of different blood tests over a lifetime, and the list keeps growing as researchers find new markers worth measuring.

What a Routine Blood Panel Already Reveals

The complete blood count, or CBC, is one of the most frequently ordered tests in medicine. It measures red blood cells, white blood cells, and platelets, along with characteristics like cell size and distribution. That sounds straightforward, but the information packed into a CBC goes well beyond checking for anemia or infection. Research has shown that various CBC components can help predict the risk of cardiovascular disease, type 2 diabetes, metabolic syndrome, and even certain cancers.¹ Ratios between different white blood cell types, for instance, have emerged as surprisingly useful markers of systemic inflammation, which underpins many chronic diseases.

A basic metabolic panel, often ordered alongside a CBC, measures blood sugar, electrolytes like sodium and potassium, and kidney function markers like creatinine. A comprehensive metabolic panel adds liver enzymes and proteins. Together, these two panels can flag kidney disease, liver damage, electrolyte imbalances, and early signs of diabetes in a single draw. They are the foundation that most other, more specialized tests build on.

Heart Disease and Cardiac Markers

Blood tests play an increasingly central role in detecting and managing heart disease. High-sensitivity troponin assays measure tiny amounts of a protein released when heart muscle is damaged. In a large community study, people with the highest detectable troponin levels had roughly double the risk of coronary heart disease compared to those with undetectable levels, about six times the risk of heart failure, and nearly four times the risk of dying from any cause.² These numbers underscore how powerful a single blood marker can be for gauging cardiac risk.

Troponin is not just useful in emergency rooms when a heart attack is suspected. Tracking troponin levels over time can also help evaluate the effectiveness of treatment. In one clinical trial, men whose troponin dropped substantially over time had very low rates of future coronary events, while those whose levels rose faced much higher risk.³ Other cardiac blood markers include B-type natriuretic peptide (BNP), which rises during heart failure, and lipid panels measuring cholesterol and triglycerides, which are fundamental to assessing long-term cardiovascular risk.

Diabetes and Thyroid Disorders

Blood tests are the primary way diabetes is diagnosed and monitored. Fasting blood glucose and oral glucose tolerance tests have been around for decades, but HbA1c has become the go-to marker for long-term blood sugar control. It reflects your average blood glucose over the preceding two to three months, making it harder to game with a single day of clean eating before a doctor visit. HbA1c has been used clinically for roughly four decades, and the research tying it to diabetes complications remains foundational to how the disease is managed.⁴

Thyroid disorders are another area where blood tests are indispensable. Measuring TSH (thyroid-stimulating hormone) alongside free T4 and total T3 can distinguish between an overactive thyroid, an underactive one, and a thyroid that is functioning normally.⁵ These tests are cheap, widely available, and often the first step when someone presents with unexplained weight changes, fatigue, or mood shifts. Thyroid antibody tests can further distinguish autoimmune thyroid conditions like Hashimoto’s disease from other causes.

Infections From Viruses to Bacteria

Blood tests detect a wide range of infectious diseases, often before symptoms become severe. For viruses like hepatitis C, the standard approach begins with an antibody screen. If antibodies are found, a follow-up test using nucleic acid amplification confirms active infection and measures the viral load, which guides treatment decisions.⁶ Similar antibody-then-confirmation strategies apply to HIV and hepatitis B. Blood screening programs using both standard antibody methods and PCR-based testing are employed around the world to detect these infections in high-risk populations.⁷

Bacterial infections pose a different challenge. Blood cultures, where a sample is incubated to see if bacteria grow, remain the gold standard for diagnosing bloodstream infections, but they can take a day or two to return results. That delay matters when someone is critically ill. Procalcitonin, a protein that spikes during bacterial infection, has shown promise as a faster indicator. In one study, procalcitonin levels were dramatically higher in patients with confirmed bloodstream infections compared to those without, and the marker was more accurate than C-reactive protein at distinguishing true infection from contamination.⁸ Another study found that using procalcitonin cut-off levels could correctly identify the vast majority of positive blood cultures and reduce unnecessary blood culture orders by roughly half.⁹ Procalcitonin is particularly sensitive for severe gram-negative bacterial infections, including those caused by antibiotic-resistant organisms.¹⁰

Autoimmune and Inflammatory Conditions

The immune system sometimes turns on the body’s own tissues, and blood tests are often how these conditions are first suspected. Autoantibodies, which are immune proteins that mistakenly target your own cells, have become a key part of diagnosing diseases like rheumatoid arthritis, lupus, scleroderma, and Sjögren’s syndrome. Each of these conditions tends to be associated with a characteristic set of autoantibodies.¹¹ When used thoughtfully, autoantibody testing helps not just with diagnosis but sometimes with prognosis and treatment decisions.¹²

Antinuclear antibodies (ANA) are among the most commonly ordered of these tests. A positive ANA result is a recognized marker for several systemic autoimmune diseases, including lupus, Sjögren’s syndrome, and mixed connective tissue disease.¹³ A caveat worth knowing: a positive ANA does not mean you have an autoimmune disease. Healthy people, particularly women and older adults, can test positive without ever developing symptoms. The test is most useful when a doctor already suspects something based on symptoms and uses the result to help narrow down the diagnosis.

Beyond autoantibodies, general inflammatory markers like erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) help track disease activity in conditions ranging from inflammatory bowel disease to vasculitis. They are not specific enough to diagnose a particular illness on their own, but rising or falling levels often tell doctors whether a treatment is working.

Cancer Screening and Monitoring

This is where expectations and reality tend to diverge the most. Blood-based tumor markers exist for many cancers, and you will see them mentioned in headlines regularly, but most of them are not reliable enough for screening healthy people. The positive predictive value for mass screening is generally poor, meaning that a high result is often a false alarm rather than a true cancer signal.¹⁴ With only a few exceptions, like calcitonin for medullary thyroid cancer and certain hormone markers for germ-cell tumors in men, established tumor markers are not considered suitable for screening asymptomatic individuals.¹⁵

Where tumor markers shine is in monitoring. Once a cancer has been diagnosed, markers like CEA for colorectal cancer, CA 19-9 for pancreatic cancer, and PSA for prostate cancer help doctors gauge how well treatment is working and catch recurrences early.¹⁴

The frontier that gets the most attention is liquid biopsy, which analyzes circulating tumor DNA, cell-free DNA, and circulating tumor cells from a standard blood draw. These technologies are already being used to identify actionable genetic alterations in tumors, predict responses to specific therapies, and monitor disease burden after surgery.¹⁶ Early cancer detection through liquid biopsy is an active area of research, though it faces real challenges. The amount of tumor-derived material circulating in early-stage cancer can be vanishingly small, and a phenomenon called clonal hematopoiesis (where blood-forming cells accumulate mutations with age, unrelated to cancer) can muddy the results, especially in older adults.¹⁶

Neurodegenerative Diseases

Until recently, diagnosing Alzheimer’s disease from blood was essentially impossible. The gold standards were expensive PET brain scans or invasive spinal taps to measure proteins in spinal fluid. That is changing. Blood-based markers, particularly a form of phosphorylated tau called p-tau217, have emerged as surprisingly accurate indicators of the amyloid plaques that define Alzheimer’s pathology. Among the various phosphorylated tau variants being studied, p-tau217 has shown the strongest performance in identifying people with amyloid buildup in the brain and in predicting early-stage disease.¹⁷

Another marker gaining traction is neurofilament light chain (NfL), a protein released when nerve fibers are damaged. Plasma NfL shows promise in distinguishing cognitive decline caused by Alzheimer’s from decline caused by other conditions, even in early stages before full-blown dementia develops.¹⁸ These blood tests are not yet standard in every clinic, but they represent a genuine shift in how neurodegenerative diseases may be diagnosed in the coming years, potentially sparing patients from invasive or expensive procedures.

Prenatal Screening and Genetic Conditions

One of the more remarkable applications of blood testing is prenatal screening. A pregnant person’s blood contains fragments of fetal DNA, and analyzing that cell-free DNA can screen for chromosomal conditions like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). In a large study of over 38,000 samples, the positive predictive value for trisomy 21 was about 98.5%, meaning that nearly all positive results were confirmed as true positives.¹⁹ Accuracy was somewhat lower for rarer conditions and sex chromosome differences, where positive predictive values ranged more widely.

Researchers are pushing cell-free DNA testing further, investigating its ability to detect smaller chromosomal deletions and duplications. This is harder, and a systematic review found that the overall positive predictive value for these smaller abnormalities was around 40%, with a false-positive rate below 0.1%.²⁰ That lower accuracy means a positive result for a microdeletion is more likely to be a false alarm than a true finding, and confirmatory diagnostic procedures are still recommended.

Beyond prenatal testing, blood-based genetic screening can identify inherited bleeding disorders in children. In one study of children with suspected inherited bleeding problems, genetic testing identified disease-causing variants in about 17% of cases overall, with a much higher rate among children with low platelet counts specifically.²¹

Nutritional Deficiencies and Vitamin Levels

Blood tests are the standard way to identify deficiencies in vitamins and minerals. Vitamin B12 is a good case study because it illustrates both the power and the pitfalls of blood-based diagnostics. The most commonly used first-line test measures total serum B12, with levels below roughly 148 pmol/L typically flagging deficiency.²² But total serum B12 has an awkward indeterminate range where levels look normal even though cells are not getting enough of the vitamin to function properly.²³

When results are borderline, doctors can order secondary tests like methylmalonic acid or homocysteine, which reflect whether the body is actually able to use its B12 stores. These markers tend to rise when functional B12 is truly low, but they can also be elevated for other reasons, complicating interpretation.²⁴ Similar layered testing approaches exist for iron deficiency (ferritin, transferrin saturation, total iron-binding capacity), vitamin D (serum 25-hydroxyvitamin D), and folate.

Allergies and Immune Reactions

When skin-prick testing is not practical or safe, blood tests that measure allergen-specific immunoglobulin E (IgE) are the primary alternative. IgE testing is currently considered the cornerstone of diagnostic evaluation for suspected allergic conditions, according to a World Allergy Organization position paper.²⁵ These tests can measure sensitization to specific allergens like pollen, pet dander, foods, or insect venom. A positive result means your immune system has produced IgE against that allergen, though it does not always mean you will have symptoms on exposure. Clinical context matters, and allergy blood tests are most useful when combined with a detailed history of symptoms.

Heavy Metals and Toxicology

Exposure to toxic heavy metals like lead, mercury, arsenic, and cadmium can be measured through blood tests, urine tests, and sometimes hair or nail samples. Blood testing is particularly important for lead exposure, where whole blood lead levels are the standard measure. For other metals, the choice between blood and urine depends on whether the exposure is recent or chronic.²⁶ Newer techniques, including dried blood spot analysis, allow measurement of multiple heavy metals from a very small blood sample, which is especially useful for population studies and wildlife monitoring.²⁷

Monitoring Drug Levels in Your Blood

Blood tests are not only used to diagnose illness but also to ensure medications are working safely. Therapeutic drug monitoring measures the concentration of specific drugs in your bloodstream to make sure levels stay within a window that is high enough to be effective but low enough to avoid toxicity. This is mainly done for medications with narrow safety margins, high variability in how people metabolize them, or effects that are hard to monitor by symptoms alone.²⁸

Common examples include certain antibiotics, anti-seizure medications, immunosuppressants used after organ transplants, and some antifungal drugs. For instance, the antifungal posaconazole shows highly variable blood concentrations between patients, so monitoring helps avoid both underexposure (which reduces effectiveness) and overexposure (which raises the risk of side effects).²⁹ Therapeutic drug monitoring is also increasingly used with certain antibiotics to make sure patients with severe infections are getting adequate doses and to slow the emergence of antibiotic resistance.³⁰

When Blood Tests Get It Wrong

For all their usefulness, blood tests are not infallible, and understanding the main sources of error can save you from unnecessary anxiety or, worse, missed diagnoses.

The most common problem is hemolysis, which is when red blood cells break open during or after the blood draw. This releases their contents into the serum and can throw off results for several common tests. Pre-analytical errors (mistakes before the sample even reaches a testing machine) account for roughly 40 to 65% of all laboratory errors, and hemolysis is responsible for about 60% of those.³¹ Potassium, LDH, and phosphorus are particularly unreliable in hemolyzed samples, while tests like creatinine and amylase remain accurate even when the sample is visibly pink or red.³¹ If you have ever been told a sample was hemolyzed and needed to be redrawn, this is why.

A subtler interference comes from biotin, a B vitamin sold in high-dose supplements for hair, skin, and nails. Standard dietary intake does not cause problems, but the mega-doses found in some supplements and in clinical trials for multiple sclerosis can interfere with immunoassays used to measure thyroid hormones, cardiac markers, and reproductive hormones. Several case reports have documented patients appearing to have Graves’ disease based on lab results that were entirely an artifact of biotin supplements. The FDA has received dozens of reports of suspected biotin interference since 2016.³² If you take a biotin supplement and your thyroid or cardiac blood work looks unexpectedly abnormal, mention the supplement to your doctor. Stopping it for a couple of days before retesting usually resolves the discrepancy.

Other interferences include heterophile antibodies (rogue antibodies in your blood that can cross-react with test components) and the high-dose hook effect, where an extremely high concentration of the substance being measured paradoxically produces a low or normal reading.³³ These are rare but clinically meaningful, and they are one reason doctors are trained to treat unexpected lab results with healthy skepticism before making major clinical decisions based on a single test.