Losing weight without trying is a symptom that is easy to brush off and hard to interpret. A study published in PLOS Medicine in September 2026 by University of Oxford researchers asked a practical question: when someone has unexplained weight loss, can the blood tests a doctor already orders, and the way those results move over time, help show who is more likely to have cancer? The answer is a cautious yes.
This article explains what the team did, what they found, what it does not prove, and what it means for how you read your own results. If you want the basics of repeated results first, read how blood test results change over time.
What the Oxford researchers studied
The team analyzed primary care records from more than 275,000 patients who had unexplained weight loss. This symptom can go along with several cancers, but also with thyroid disease, diabetes, digestive conditions and depression. Roughly 14,000 of these patients were diagnosed with cancer within six months.
The researchers looked at 26 blood tests commonly used in primary care. They compared how results differed, and how they changed over time, between patients who were and were not later diagnosed with cancer. It was a retrospective cohort study, which means it looked back at existing records rather than following new volunteers.
Several of these tests, such as white blood cells, red blood cells, hematocrit and platelets, belong to a routine blood count. To see what each line means, read our complete blood count guide.
What they found
Of the 26 tests, 23 showed trends linked to overall cancer risk. Some patterns pointed to specific cancers:
- Changes in white blood cell and neutrophil counts were linked to lung cancer.
- Changes in red blood cell count, hematocrit and the platelet-to-lymphocyte ratio were linked to prostate cancer.
- Several trend-based findings identified cancer more effectively than a single abnormal result once the patient’s age and sex were taken into account.
Lead author Brian Nicholson summed up one practical point: results become more accurate for cancer when age and sex are added, a calculation a laboratory could easily perform.
Single result, adjusted result, trend
| Approach | What it looks at | Main strength | Main limit |
|---|---|---|---|
| Single result | Is today’s value above or below the laboratory reference range? | Simple and available everywhere | Can miss changes that stay inside the normal range |
| Result adjusted for age and sex | The same value, interpreted for the person’s age and sex | Closer to the individual | Still a snapshot in time |
| Trend over time | How a value moves across several tests | Uses your own baseline | Needs repeated tests and good records |
Why unexplained weight loss deserves attention
Mayo Clinic describes unintentional loss of more than 5% of body weight over 6 to 12 months as a concern, which is about 8 pounds for a 160-pound adult. MedlinePlus gives a similar threshold of more than 10 pounds (4.5 kg) or 5% of normal body weight. Older adults and people with existing conditions may need an evaluation even after smaller losses.
Causes other than cancer
Cancer is only one possible explanation. Reference sources list many others:
- An overactive thyroid or undiagnosed diabetes.
- Digestive conditions such as celiac disease, Crohn’s disease or ulcerative colitis.
- Depression, anxiety and loss of appetite.
- Chronic infections and some medications.
A typical evaluation combines a physical exam, a detailed history, blood and urine tests and, when needed, imaging. If early results are inconclusive, doctors sometimes suggest watchful waiting for one to six months with a food diary.
Which blood tests are usually part of the work-up
The exact list depends on your history and symptoms. These are common examples:
| Test | What it can reflect |
|---|---|
| Complete blood count | Anemia, infection, inflammation, platelet changes |
| Blood glucose and HbA1c | High blood sugar, a possible sign of diabetes |
| Thyroid tests | An overactive thyroid |
| Ferritin and iron markers | Iron deficiency or storage problems |
For two of these, see how to read your HbA1c results and what ferritin levels mean.
Latest scientific advances
The Oxford study fits into a growing line of research on blood test trends. Here is what recent reviews and studies add, in plain language.
Trends can matter even inside the normal range
A 2024 systematic review pooled studies on blood test trends and undiagnosed cancer. The tests studied most often were hemoglobin, C-reactive protein and fasting glucose, and the cancers studied most often were pancreatic and colorectal. Rising or falling trends in some tests were linked to several cancer types, but for many test and cancer combinations, evidence was limited or absent. What it changes for you: a result can be within the reference range and still be worth discussing if it has moved a lot from your own baseline.
Prediction tools exist, but are not ready for everyone
A 2025 review examined models that use blood test trends to estimate cancer risk. Most relied on the full blood count and were built mainly for colorectal cancer. The best known of them performed reasonably well in outside validation, yet many models were rarely tested thoroughly. What it changes for you: these tools may one day help doctors decide who needs further tests, but they are not diagnostic tests.
Signals can appear months before a diagnosis
A 2025 UK case-control study of people with vague abdominal symptoms found that abnormal blood results became more frequent in the months before a cancer diagnosis, with raised inflammatory markers the most common abnormality. What it changes for you: persistent symptoms plus unusual results justify a conversation with your doctor, not a conclusion on your own.
What this study does not show
This is one retrospective study in patients who already had unexplained weight loss, so its findings do not apply to everyone who has a blood test. The summaries we reviewed do not report detailed statistical measures such as predictive values, and the authors plan to examine other vague symptoms, such as fatigue and vomiting. Blood test trends do not diagnose cancer. At most, they may help doctors prioritize who should be investigated further.
How to use your own results
Keep copies of past blood test reports and note the date and laboratory for each one. When you receive a new report, compare it with the previous ones rather than looking only at the flags for out-of-range values. If a value has moved clearly in one direction, or you have lost weight without trying, bring the reports to your appointment.
When to talk to your doctor
- You have lost more than 5% of your body weight over 6 to 12 months without trying.
- Weight loss comes with fever, night sweats, bone pain, excessive thirst or a persistent cough.
- You notice blood in stool, a lasting change in bowel habits or difficulty swallowing.
- Your results have changed noticeably between two tests, even if each stays within range.
Do not wait for your next routine visit if several of these apply. Seek urgent care for severe symptoms.
Glossary
- Retrospective cohort study: a study that looks back at existing records to compare groups over time.
- Hematocrit: the share of your blood volume made up of red blood cells.
- Neutrophils: the most common white blood cells, first responders to infection.
- Platelet-to-lymphocyte ratio: platelet count divided by lymphocyte count, a simple inflammation-related index.
- Reference range: the interval of values seen in most healthy people, set by each laboratory.
FAQ
Can a blood test tell me if unexplained weight loss is cancer?
No. Blood tests can show clues such as anemia, inflammation or abnormal blood sugar, but they do not diagnose cancer. Your doctor decides which further exams are needed.
How much weight loss is considered unexplained?
Mayo Clinic and MedlinePlus flag more than 5% of body weight over 6 to 12 months, or more than 10 pounds, when the loss is not intentional.
Are normal blood test results reassuring?
Often, but not always. Researchers are studying whether changes that stay inside the normal range carry information. If symptoms persist, tell your doctor even when results are normal.
Which blood tests change before a cancer diagnosis?
Studies point to white blood cells, red blood cells, platelets and inflammatory markers, among others. The pattern differs between cancers and between people, so no single test can be relied on.
Sources
- EurekAlert: Blood test trends may help identify patients at increased risk of cancer (University of Oxford, PLOS Medicine, September 2026)
- PLOS Medicine, DOI 10.1371/journal.pmed.1004956
- Mayo Clinic: Unexplained weight loss, when to see a doctor
- MedlinePlus: Weight loss, unintentional
- Cleveland Clinic: Unexplained weight loss, causes and testing
- The Association between Blood Test Trends and Undiagnosed Cancer: A Systematic Review and Critical Appraisal (Virdee et al., 2024, Cancers)
- Clinical Prediction Models Incorporating Blood Test Trend for Cancer Detection (Virdee et al., 2025, JMIR Cancer)
- A UK population-based case-control study of blood tests before cancer diagnosis in patients with non-specific abdominal symptoms (Rafiq et al., 2025, British Journal of Cancer)
Other articles
- Compare what a CBC and a CMP each measure
- Follow the 2026 update on multi-cancer early detection tests
- Understand what FDA approval of a colon cancer blood test means
- Review what new research shows on lung cancer blood tests
Understand your own blood test results with AI DiagMe
Comparing several reports is easier with help. Understand your lab results with AI DiagMe, an AI reviewed by a committee of physicians, with data hosted in France.
