- 01A reference range describes healthy people; a diagnostic cutoff describes disease
The range printed beside a result is the middle 95% of healthy people, so falling outside it is a statistical flag. Guideline cutoffs such as HbA1c 6.5% or eGFR under 60 for 3 months come from outcome studies and are what diagnoses rest on.
A blood test can show diabetes and prediabetes, anemia, iron and vitamin deficiencies, thyroid disease, high cholesterol, kidney and liver problems, inflammation and some infections, but only for the markers the lab was asked to measure. What shows up in a blood test ordered at a checkup is usually blood cells, kidney and liver chemistry, and cholesterol. Some useful tests have to be requested by name: ferritin (your iron stores), ApoB and Lp(a) (two extra heart-risk markers), vitamin D and thyroid antibodies. Below is each condition, the marker that shows it, the cutoff that counts as off, and where blood goes blind.
What's in your blood that a lab can measure
Blood is a transport system, so it carries a sample of almost everything the body makes, burns or throws away. Labs group what they measure into six families.
- Cells: red cells, white cells and platelets, counted in a complete blood count (CBC, called a full blood count or FBC in the UK, part of the haematology profile). The CBC shows anemia, signs of infection and some blood disorders. See the CBC guide.
- Fuel and fats: glucose and HbA1c (average blood sugar over about 3 months), plus cholesterol and ApoB, the particle count behind heart risk. See how to read your lipid panel.
- Proteins and enzymes (biochemistry): albumin, and the liver enzymes ALT, AST and ALP, which leak into blood when liver cells are stressed. See liver enzymes explained.
- Hormones (endocrinology): thyroid (TSH, free T4), sex hormones and cortisol. See how to read your thyroid panel.
- Waste (biochemistry): creatinine, urea and bilirubin, which rise when the kidneys or liver fall behind on clearing them. See kidney function explained.
- Immune signals (immunology): CRP, a general inflammation marker, and antibodies, which show past infection, vaccination or autoimmune disease. See inflammation markers explained and chronic inflammation.
Five markers that cover the most ground
Conditions a blood test can show, and the marker that shows each
The table below maps common conditions to the marker that shows them and the cutoff that counts as off. A result past a cutoff is a reason for a conversation with a clinician and often a repeat test; one reading alone rarely settles a diagnosis.
| Condition | Marker(s) | What counts as off |
|---|---|---|
| Type 2 diabetes | HbA1c, fasting glucose | HbA1c 6.5% or higher, or fasting glucose 126 mg/dL or higher (ADA) |
| Prediabetes | HbA1c, fasting glucose | HbA1c 5.7 to 6.4%, or fasting glucose 100 to 125 mg/dL (ADA) |
| Anemia | Hemoglobin, MCV (red cell size) for the type | Hemoglobin under 13.0 g/dL in men, under 12.0 g/dL in non-pregnant women |
| Iron deficiency | Ferritin | Under 15 µg/L in healthy adults; under 70 µg/L when inflammation is present (WHO) |
| Hypothyroidism or hyperthyroidism | TSH, then free T4 | TSH outside the lab range, read with free T4 |
| High cholesterol and heart risk | LDL, ApoB, Lp(a) | LDL and ApoB targets depend on overall risk; Lp(a) above about 50 mg/dL (125 nmol/L) counts as raised |
| Heart attack, in an emergency | High-sensitivity troponin | A rise above the 99th percentile of healthy people, with a change between two draws |
| Chronic kidney disease | eGFR (kidney filtering estimate) plus urine ACR (albumin leaking into urine) | eGFR under 60, or ACR 30 mg/g or more, for over 3 months (KDIGO) |
| Fatty liver | ALT, AST, GGT, platelets | ALT can be normal; FIB-4, a score built from age, AST, ALT and platelets, estimates scarring risk |
| Pancreas problems | Lipase, amylase | Lipase over 3 times the upper limit, with typical upper belly pain, points to acute pancreatitis; HbA1c reflects insulin output only indirectly, through blood sugar |
| B12 deficiency, including pernicious anemia, and vitamin D deficiency | Vitamin B12, vitamin D 25-OH | Below the lab range; borderline B12 is often checked with MMA (methylmalonic acid), which rises when B12 is truly short |
| Magnesium deficiency | Serum magnesium | Can read normal while body stores are low, because under 1% of the body's magnesium is in blood |
| Celiac disease | tTG-IgA, a gluten antibody, with total IgA to check the test can work | Positive tTG-IgA; only reliable while still eating gluten, so talk to a clinician before cutting gluten out |
| Gout risk | Uric acid | High uric acid; many people with high levels never get gout |
| Inflammation | CRP | Raised CRP says something is inflamed, not what. See chronic inflammation. |
| Infections | HIV test (HIV Ag/Ab), hepatitis C antibody (anti-HCV), hepatitis B surface antigen (HBsAg) | Any reactive result needs a confirmatory test |
The diabetes cutoffs come from the American Diabetes Association: an HbA1c of 6.5% or higher, or a fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or higher, meets the diagnostic threshold, confirmed by a second abnormal result unless symptoms are obvious 1. The ferritin cutoff moves with inflammation because ferritin is also an inflammation protein: WHO puts iron deficiency below 15 µg/L in healthy adults but below 70 µg/L in someone with an infection or inflammatory illness 2. The full comparison of the two diabetes tests is on A1c vs fasting glucose.
Where an example HbA1c falls%
Heavy thirst, frequent urination, unexplained weight loss, vomiting or confusion alongside high blood sugar needs care the same day, whatever your last HbA1c showed. See always thirsty.
Kidney disease is the clearest case where blood alone misses early damage. KDIGO defines chronic kidney disease as an eGFR below 60 mL/min/1.73 m², or a urine albumin-to-creatinine ratio (ACR) of 30 mg/g or more, lasting more than 3 months 3. A person can have a normal eGFR and still be leaking albumin into urine, which only a urine test shows.
CalculatoreGFR calculatorTurn a creatinine result, age and sex into an eGFR and see which kidney stage it falls in.Work it out Can a blood test detect cancer?
A routine blood panel is not a cancer test. A CBC can point toward leukemia or lymphoma through very abnormal white cell, red cell or platelet counts, and an unexplained high calcium or abnormal liver result can prompt a closer look, but most solid tumors leave a standard panel normal in their early stages.
Tumor markers such as PSA, CA-125 and CEA are proteins that some cancers release. They are used mainly to monitor a cancer already diagnosed, because each one also rises with harmless conditions: PSA with an enlarged prostate or infection, CA-125 with endometriosis or a period. Screening guidance reflects that. The US Preventive Services Task Force recommends against PSA screening for men 70 and older 4. For men 55 to 69, it says:
For men aged 55 to 69 years, the decision to undergo periodic prostate-specific antigen (PSA)-based screening for prostate cancer should be an individual one.US Preventive Services Task Force, Prostate Cancer: Screening, 2018
CA-125 is not used to screen women without symptoms or a high genetic risk, because it raises too many false alarms in healthy women. For each marker, see tumor markers explained.
Multi-cancer early detection tests look for tumor DNA in blood. In the first PATHFINDER study of 6,621 adults aged 50 and over, the test found a cancer signal in 1.4% of people, and 38% of those with a signal turned out to have cancer 5. The larger limit is what these tests miss: many cancers, especially early ones, give no signal, so a negative result cannot replace guideline screening such as a mammogram, a cervical screen or a colonoscopy. More on the Galleri test.
What a blood test can't tell you
Blood shows chemistry and cells, so a problem that is mainly structural or electrical usually leaves it unchanged. Common blind spots:
- Structural problems: a hernia, a slipped disc, a torn ligament, gallstones, and most small early tumors. These need imaging.
- Narrowed arteries: cholesterol and ApoB show risk, but only a coronary calcium score or other imaging shows plaque already there.
- Mental health and most brain conditions: depression, anxiety and migraine have no blood marker, though a test can rule out thyroid disease or anemia behind the symptoms.
- Early kidney damage that shows only in urine, picked up by a urine ACR or urinalysis.
- Conditions with swinging hormones: estradiol and FSH change from day to day in perimenopause, so perimenopause is usually diagnosed from symptoms and cycle changes. See how cycle hormones change.
Every blood test is also a single moment. A ferritin drawn during a cold, or a glucose after a stressful week, reads differently from the same marker a month later, which is why trends beat single readings.
Why a healthy person can get an abnormal result
The range printed beside a result is the middle 95% of healthy people, so falling outside it is a statistical flag. Guideline cutoffs such as HbA1c 6.5% or eGFR under 60 for 3 months come from outcome studies, and those are what diagnoses rest on.
Because each range leaves out 5% of healthy people by design, 1 healthy person in 20 lands outside any single range. Across 20 markers, the chance that a healthy person sees at least one flag is about 64% (1 minus 0.95 to the 20th power). Real panels contain linked markers, so the true figure runs a little lower, but one flag on a long panel is still common and often means nothing. See reference ranges explained.
Everyday things also push a healthy result past a line:
- Not fasting, which raises glucose and triglycerides. See fasting before a blood test and coffee before a blood test.
- A hard workout in the 2 days before the draw, which raises the muscle enzyme CK and AST.
- A cold or other infection, which raises CRP and ferritin.
- Dehydration, which concentrates hemoglobin, albumin and creatinine.
- Time of day: cortisol and testosterone are highest in the morning.
- Biotin supplements, which can throw off thyroid and troponin tests; tell whoever orders the test if you take one.
The US Food and Drug Administration warned about biotin in a safety communication 6:
Biotin in blood or other samples taken from patients who are ingesting high levels of biotin in dietary supplements can cause clinically significant incorrect lab test results.US Food and Drug Administration, Safety Communication on biotin interference, 2019 update
The supplement most likely to skew a result
When one marker sits slightly outside its range, the usual next steps are to repeat it under the same conditions, compare it with past results, and look for a pattern across related markers. Other supplements that shift results are covered in supplements that affect blood tests.
Standard panel vs extra tests
| Usually in a routine panel | Usually has to be requested |
|---|---|
| CBC (red cells, white cells, platelets) | Ferritin and iron studies |
| Metabolic panel (glucose, kidney, liver, electrolytes) | ApoB and Lp(a) |
| Lipid panel (LDL, HDL, triglycerides) | Vitamin D, vitamin B12, thyroid antibodies |
| Sometimes HbA1c and TSH | hs-CRP, fasting insulin, sex hormones |
Two infection screens are advised in the US for adults whether or not they feel ill: an HIV test at least once for everyone aged 13 to 64, and a hepatitis C test at least once for every adult 18 and over. Neither is part of a routine panel unless asked for. Lp(a) is worth one request too: it is mostly inherited, so a single reading in adult life is usually enough. The ApoB vs LDL comparison explains why ApoB is worth asking for.
Markers to read with it
Do you need symptoms to get tested?
No. Prediabetes, high ApoB, early kidney decline and fatty liver are usually silent for years, and blood is often the only place they show before damage builds. Common reasons to test while feeling well include a family history of diabetes or early heart disease, setting a baseline, checking the effect of a new diet or medication, and planning a pregnancy. How often to repeat depends on the first result. A normal panel can usually wait a year or more, a prediabetes-range HbA1c is usually retested at least once a year, and a low eGFR or raised ACR is rechecked after about 3 months to see whether it has lasted.
Got results? How to read them
- 1Check the units and the rangeThe same marker reads differently in mg/dL and mmol/L, and each lab prints its own range.
- 2Read the pattern, not one flagLow hemoglobin with low ferritin and low MCV tells a different story from low hemoglobin with a high MCV. Some patterns are read as ratios; see blood test ratios.
- 3Compare against last timeA ferritin falling from 90 to 30 over two years says more than either number.
- 4Bring flagged results to a clinicianEspecially several flags in the same organ group. Ask whether the flag should be repeated, and whether a related test would settle it: ferritin for low hemoglobin, urine ACR for a borderline eGFR, FIB-4 for a raised ALT. If the lab marks a result critical or calls you about it, contact a clinician the same day.
Lab-specific guides cover reading Quest results, reading Labcorp results and getting your results from the patient portal. For men choosing what to test, see blood tests for men; if you are thinking about supplements after a result, read supplements based on your blood test first.
Citations
- American Diabetes Association. Standards of Care in Diabetes 2025, section 2: Diagnosis and classification of diabetes. Diabetes Care 2025;48(Suppl 1).
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020.
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int 2024;105(4S).
- US Preventive Services Task Force. Prostate Cancer: Screening. JAMA 2018;319(18):1901-1913.
- Schrag D, et al. Blood-based tests for multicancer early detection (PATHFINDER): a prospective cohort study. Lancet 2023;402:1251-1260.
- US Food and Drug Administration. UPDATE: The FDA warns that biotin may interfere with lab tests: FDA Safety Communication. 2017, updated 5 November 2019 (archived copy).
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
