A curated reference for understanding blood tests, biomarkers, symptoms, and lab interpretation — built from clinical guidelines, peer-reviewed research, and authoritative medical sources.
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- Why This Exists
- Part 1 — Symptoms & What to Test For
- Part 2 — Blood Test Panels
- Part 3 — Understanding Your Results
- Part 4 — Testing Options
- Part 5 — Authoritative References
- Contributing
Most people receive lab results as a wall of numbers with no context. Reference ranges vary by lab, age, and sex. Symptoms rarely map cleanly to a single test. And the gap between "within range" and "optimal" is rarely explained.
This guide collects the most useful starting points — from symptom-to-test mappings through to the clinical guidelines that underpin what the numbers actually mean.
The most common reason people get blood tests is to investigate a symptom. The challenge: most symptoms have multiple potential causes, each requiring different markers. The sections below map common symptoms to their most clinically relevant tests.
Fatigue is the most investigated symptom in primary care and one of the most difficult to diagnose from a single marker. The key panels to request:
Primary markers:
- Ferritin — the most sensitive marker of iron status; low ferritin causes fatigue even when haemoglobin is normal
- TSH — thyroid-stimulating hormone; both hypothyroidism and hyperthyroidism cause fatigue
- Haemoglobin / CBC — rules out anaemia
- HbA1c / Fasting Glucose — dysregulated blood sugar is a leading and under-diagnosed cause of fatigue
- Vitamin D (25-OH) — deficiency is extremely common and directly linked to low energy
- Vitamin B12 — deficiency causes neurological fatigue and is reversible
- CRP / ESR — elevated inflammation markers suggest an underlying cause
Secondary (if primary panel is normal):
- Cortisol (morning, fasting) — adrenal dysfunction
- Free T3 / Free T4 — deeper thyroid function if TSH is borderline
- Fasting insulin — insulin resistance causes fatigue long before HbA1c rises
- Magnesium — intracellular deficiency rarely shows in standard panels
Useful resource: What blood tests should I get for fatigue? — detailed symptom-to-test guide with clinical context
Clinical references:
- Fatigue — MedlinePlus (NIH)
- Approach to the patient with fatigue — UpToDate overview
- Iron deficiency without anaemia — BMJ 2021
Cognitive symptoms are frequently metabolic or nutritional in origin before psychiatric causes are considered.
Key markers:
- Vitamin B12 — deficiency causes cognitive slowing and neurological symptoms
- Folate — works with B12; deficiency impairs methylation
- TSH / Free T4 — hypothyroidism commonly presents as cognitive symptoms
- HbA1c / Fasting Glucose — glucose dysregulation directly impairs cognition
- Ferritin — iron is required for dopamine synthesis
- Vitamin D — associated with cognitive function across multiple cohort studies
- Homocysteine — elevated levels are associated with cognitive decline and B-vitamin deficiency
Clinical references:
Hair loss has many causes. Blood tests rule out the most common reversible ones before considering genetic or hormonal aetiology.
Key markers:
- Ferritin — hair loss can occur with low-normal ferritin even without anaemia; target >70 µg/L for hair health
- TSH / Free T3 / Free T4 — thyroid dysfunction (both hypo and hyper) causes hair loss
- Zinc — deficiency impairs hair follicle cycling
- Vitamin D — receptors in hair follicles; deficiency linked to alopecia areata
- CBC — anaemia itself can cause diffuse shedding
- DHEA-S / Testosterone / Free Androgen Index — for pattern hair loss (androgenic aetiology)
- Biotin — rarely deficient but worth ruling out
Clinical references:
- Hair loss — Mayo Clinic
- Diffuse alopecia and nutritional deficiencies — Dermatology Practical & Conceptual 2017
Mood symptoms are commonly driven by metabolic and nutritional factors that respond to targeted treatment.
Key markers:
- TSH — hyperthyroidism mimics anxiety; hypothyroidism causes low mood
- Vitamin D — deficiency is associated with depression in multiple meta-analyses
- Vitamin B12 / Folate — low levels impair serotonin and dopamine synthesis
- Ferritin — iron is required for neurotransmitter production
- Magnesium — plays a role in GABA regulation; deficiency associated with anxiety
- Cortisol (morning) — HPA axis dysregulation affects mood and stress response
- Fasting Glucose / HbA1c — blood sugar instability directly affects mood
Clinical references:
- Vitamins and depression — Journal of Psychiatry and Neuroscience
- Thyroid disorders and mental health — Thyroid journal (ATA)
Weight gain — key markers:
- TSH / Free T4 — hypothyroidism is the first exclusion
- Fasting Insulin / HOMA-IR — insulin resistance promotes fat storage
- HbA1c / Fasting Glucose
- Cortisol — elevated cortisol promotes central adiposity
- Testosterone / DHEA-S — low androgens reduce metabolic rate in men and women
Weight loss — key markers:
- TSH — hyperthyroidism dramatically increases metabolic rate
- CBC / CRP — unintentional weight loss warrants screening for infection or malignancy
- HbA1c — uncontrolled Type 1/2 diabetes
- Albumin — marker of nutritional status
Clinical references:
Key markers:
- CRP (high sensitivity) — acute and chronic inflammation
- ESR (erythrocyte sedimentation rate) — non-specific but sensitive inflammation marker
- Uric Acid — gout screening
- Rheumatoid Factor (RF) + Anti-CCP — rheumatoid arthritis
- ANA (antinuclear antibody) — autoimmune screening
- Vitamin D — deficiency is associated with musculoskeletal pain
- Ferritin — elevated in inflammatory states (acute phase reactant)
Clinical references:
Cold intolerance as an isolated symptom almost always points to thyroid or iron dysfunction.
Key markers:
- TSH / Free T4 / Free T3 — hypothyroidism is the primary cause
- Ferritin / Haemoglobin — anaemia reduces tissue oxygenation and heat production
- Fasting Glucose — poor glucose regulation impairs thermogenesis
Key markers:
- CBC with differential — white cell count and differential patterns
- Zinc — essential for innate and adaptive immunity
- Vitamin D — plays a direct role in immune regulation
- Vitamin C (plasma) — depleted rapidly during infection
- IgA / IgG / IgM — immunoglobulin levels if recurrent infections suggest primary immunodeficiency
Clinical references:
Key markers:
- Magnesium — required for muscle contraction and ATP production
- Potassium — hypokalaemia causes profound muscle weakness
- Calcium — hypocalcaemia causes muscle cramps and weakness
- Vitamin D — deficiency causes proximal myopathy
- TSH — both hypo and hyperthyroidism cause muscle dysfunction
- CK (creatine kinase) — elevated in muscle breakdown (myopathy, rhabdomyolysis)
Key markers:
- Total and Free Testosterone — the primary driver in men and women
- SHBG (sex hormone binding globulin) — affects bioavailable testosterone
- LH / FSH — pituitary hormones driving testosterone production
- Prolactin — elevated prolactin suppresses gonadal axis
- TSH — thyroid dysfunction directly affects libido
- Ferritin — chronic iron deficiency reduces energy and hormonal function
The perimenopause transition produces symptoms — fatigue, brain fog, mood changes, weight gain, hair loss — that closely overlap with thyroid and nutritional deficiencies. Testing both simultaneously avoids misattribution.
Key markers:
- FSH — rises in perimenopause; >10 IU/L suggests early transition, >25 IU/L menopause
- Oestradiol (E2) — fluctuates significantly; single result has limited value
- AMH (anti-Müllerian hormone) — ovarian reserve indicator
- TSH / Free T4 — thyroid disease peaks in perimenopausal women
- Ferritin — heavy perimenopausal periods cause iron depletion
- Vitamin D — critical for bone health during oestrogen decline
- HbA1c — insulin resistance increases post-menopause
- Lipid panel — cardiovascular risk rises significantly post-menopause; AHA recommends monitoring
Clinical references:
- Menopause — NICE guideline NG23
- Cardiovascular risk in menopause — AHA Scientific Statement 2020
- Perimenopause — The Menopause Society (formerly NAMS)
The CBC measures the cellular components of blood. Ordered in virtually every workup.
| Marker | What it measures | Key clinical significance |
|---|---|---|
| RBC | Red blood cell count | Anaemia, polycythaemia |
| Haemoglobin | Oxygen-carrying protein | Anaemia severity |
| Haematocrit | % of blood that is RBCs | Hydration status, anaemia |
| MCV | Mean red cell volume | Distinguishes iron vs B12/folate deficiency |
| MCH | Haemoglobin per red cell | Anaemia type |
| Platelets | Clotting cells | Bleeding risk, bone marrow function |
| WBC | White blood cell count | Infection, immune function |
| Neutrophils | Bacterial infection fighter | Elevated in bacterial infection |
| Lymphocytes | Viral immunity | Elevated in viral infection |
| Eosinophils | Allergy / parasite response | Elevated in allergy, asthma |
Reference: CBC — Lab Tests Online (AACC)
14-test panel covering kidney function, liver function, electrolytes, and blood sugar.
| Marker | Normal range (adult) | Significance |
|---|---|---|
| Glucose (fasting) | 70–99 mg/dL | Diabetes screening |
| BUN | 7–20 mg/dL | Kidney function, hydration |
| Creatinine | 0.6–1.2 mg/dL (M); 0.5–1.1 (F) | Kidney filtration |
| eGFR | >60 mL/min/1.73m² | Chronic kidney disease staging |
| Sodium | 136–145 mEq/L | Fluid balance |
| Potassium | 3.5–5.0 mEq/L | Cardiac and muscle function |
| ALT | 7–56 U/L | Liver inflammation |
| AST | 10–40 U/L | Liver and muscle damage |
| Albumin | 3.5–5.0 g/dL | Nutritional status, liver function |
| Total Bilirubin | 0.1–1.2 mg/dL | Liver and red cell breakdown |
| ALP | 44–147 U/L | Liver, bone |
| Calcium | 8.5–10.2 mg/dL | Bone, parathyroid, neuromuscular |
Ranges are approximate and vary by laboratory and sex. Always interpret against your lab's specific reference range.
Reference: CMP — Mayo Clinic
| Marker | Optimal | Borderline | High Risk |
|---|---|---|---|
| Total Cholesterol | <200 mg/dL | 200–239 | ≥240 |
| LDL-C | <100 mg/dL | 130–159 | ≥160 |
| HDL-C (M) | >40 mg/dL | 40–59 | <40 (low = risk) |
| HDL-C (F) | >50 mg/dL | 50–59 | <50 (low = risk) |
| Triglycerides | <150 mg/dL | 150–199 | ≥200 |
| Non-HDL Cholesterol | <130 mg/dL | — | ≥190 |
ACC/AHA 2019 Guideline thresholds. Requires fasting sample for accurate triglycerides.
Clinical guideline: ACC/AHA 2019 Guideline on Primary Prevention of Cardiovascular Disease
| Marker | Reference range | Notes |
|---|---|---|
| TSH | 0.4–4.0 mIU/L | First-line test; abnormal TSH triggers T4/T3 |
| Free T4 | 0.8–1.8 ng/dL | Active thyroid hormone |
| Free T3 | 2.3–4.2 pg/mL | Most active form; conversion from T4 |
| Anti-TPO | <35 IU/mL | Hashimoto's antibody |
| Anti-thyroglobulin | <40 IU/mL | Autoimmune thyroid disease |
TSH has a circadian rhythm; collect in the morning for consistency. Pregnancy requires tighter targets.
Clinical guideline: ATA Guidelines for Hypothyroidism in Adults (2014)
| Marker | Reference range | Significance |
|---|---|---|
| Serum Iron | 60–170 µg/dL | Snapshot of circulating iron |
| Ferritin | 12–300 µg/L (M); 12–150 (F) | Best marker of iron stores |
| TIBC | 240–450 µg/dL | Rises in iron deficiency |
| Transferrin Saturation | 20–50% | <20% suggests depletion |
| Haemoglobin | — | Late-stage iron deficiency indicator |
Ferritin is an acute phase reactant — inflammation elevates ferritin even in iron-deficient states, masking depletion.
Reference: Iron deficiency — WHO Global Anaemia Prevalence
| Marker | Use case |
|---|---|
| hsCRP | Cardiovascular risk stratification; general inflammation |
| CRP (standard) | Acute infection and inflammation |
| ESR | Non-specific; sensitive for inflammatory conditions |
| Ferritin | Elevated in inflammation (acute phase reactant) |
| IL-6 | Research context; upstream of CRP |
| Fibrinogen | Cardiovascular risk; clotting |
Reference: hsCRP and cardiovascular risk — AHA Scientific Statement
Adrenal:
- Cortisol (morning, fasting 8am) — HPA axis function
- DHEA-S — adrenal androgen; declines with age
Reproductive (men):
- Total Testosterone — requires morning fasting sample
- Free Testosterone — bioavailable fraction; calculated or measured
- SHBG — affects bioavailable testosterone
- LH / FSH — pituitary signalling
- PSA — prostate screening (≥50 years, or ≥40 with family history per ACS)
Reproductive (women):
- Oestradiol (E2), Progesterone — cycle-day dependent; requires timed collection
- AMH — ovarian reserve; cycle-independent
- LH / FSH — ovulatory function, menopause staging
- Prolactin — elevated in amenorrhoea or galactorrhoea
Clinical guideline: Endocrine Society Clinical Practice Guidelines
| Nutrient | Test | Optimal range | Notes |
|---|---|---|---|
| Vitamin D | 25-OH Vitamin D | 40–60 ng/mL | Most labs flag <20 as deficient; 30–100 is "sufficient" |
| Vitamin B12 | Serum B12 | >400 pg/mL | Serum B12 is insensitive; methylmalonic acid is more accurate |
| Folate | Serum or RBC folate | >4 ng/mL | RBC folate better reflects long-term status |
| Magnesium | Serum magnesium | 0.85–1.10 mmol/L | Serum reflects only 1% of body stores — poor marker of true status |
| Zinc | Serum zinc | 70–120 µg/dL | Fasting morning sample; inflammation suppresses levels |
| Iron | See Iron Studies above | — | — |
Every lab report contains three elements that require interpretation together:
- Your result — the measured value
- The reference range — the lab's population-based interval (typically the middle 95% of a healthy population)
- Clinical context — your symptoms, history, medications, and other results
Key principle: A result within the reference range is not necessarily optimal, and a result slightly outside the range is not necessarily pathological. Reference ranges are statistical constructs, not clinical thresholds.
Useful guides:
- How to Read Blood Test Results — Lab Tests Online (AACC)
- Understanding laboratory results — NHS (UK)
Reference ranges are established from large population samples and represent the middle 95% of results. This means:
- 5% of perfectly healthy people will have "abnormal" results by definition
- A result at the bottom of the range (e.g., ferritin of 13 µg/L when the range is 12–300) may be suboptimal even though it's technically "normal"
- Sex, age, fasting status, time of day, and lab methodology all affect results
Organisations publishing evidence-based optimal ranges:
- American Diabetes Association — Standards of Care
- American Thyroid Association
- ACC/AHA Cardiovascular Guidelines
- NICE Clinical Guidelines (UK)
Clinical calculators:
| Calculator | Use | Source |
|---|---|---|
| eGFR Calculator | Estimates kidney filtration rate from creatinine, age, and sex using the CKD-EPI equation | FixFirst — free, no account |
| LDL Friedewald Calculator | Calculates LDL-C from total cholesterol, HDL, and triglycerides | MDCalc |
| ASCVD Risk Calculator | 10-year cardiovascular risk (ACC/AHA pooled cohort equation) | ACC/AHA |
| HOMA-IR Calculator | Insulin resistance from fasting glucose and insulin | MDCalc |
| CKD-EPI eGFR | KDIGO-endorsed kidney function calculator | NKF |
| Cardiovascular Risk — QRISK3 | UK-validated 10-year CVD risk | QRISK |
| TSH + thyroid hormone converter | Unit conversions for thyroid labs | ATA |
Reference databases:
- Lab Tests Online — AACC patient education resource; explains each test, normal ranges, and when it's ordered
- MedlinePlus Medical Tests — NIH-maintained test library
- NHS A–Z of conditions — UK clinical context for each test
- KDIGO Guidelines — Kidney disease staging and eGFR interpretation
Self-collection kits — typically finger-prick capillary samples — that allow testing without a GP visit. Useful for monitoring over time or testing markers a GP may not order.
| Service | Coverage | Notes |
|---|---|---|
| Everlywell | USA | Wide panel range; CLIA-certified labs |
| LetsGetChecked | USA, UK, Ireland | Nurse-reviewed results |
| Medichecks | UK | Very wide panel options; GP letter available |
| Thriva | UK | Subscription model; trend tracking |
| SiPhox Health | USA | High-sensitivity CRP, lipids, hormones |
| Paloma Health | USA | Thyroid-specialised |
| imaware | USA | Autoimmune, cardiovascular focus |
All at-home tests use self-collected capillary (finger-prick) blood unless otherwise stated. Venous samples from a clinic are generally more accurate for most markers.
Walk-in access to venous blood draws, often at a lower cost than through insurance.
| Service | Coverage | Notes |
|---|---|---|
| Quest Diagnostics | USA | Largest US lab network |
| LabCorp | USA | Direct access testing in most states |
| Any Lab Test Now | USA | Franchise walk-in model |
| Walk-In Lab | USA | Online ordering; LabCorp / Quest network |
| Medichecks Clinic | UK | In-clinic venous draw option |
Tools that interpret uploaded lab results and provide context for individual markers.
| Tool | Cost | Differentiator |
|---|---|---|
| FixFirst | Free | Ranks top 3 priorities by clinical impact; sex- and age-adjusted; guidelines-anchored; no account, PDF discarded after analysis |
| Docus | Freemium | AI second opinion; broader medical Q&A |
| Kantesti | Paid | Detailed biomarker education |
| Bloodwork Lab Interpreter (ChatGPT) | Freemium | General-purpose; not calibrated to clinical guidelines |
| SiPhox AI | Bundled with testing | Paired with their own testing service |
AI tools are not a substitute for clinical interpretation by a qualified practitioner. Use for education and as a starting point for conversations with your doctor.
The gold standard for interpreting what individual biomarker levels actually mean clinically.
- ADA Standards of Medical Care in Diabetes — diabetesjournals.org/care — Glucose, HbA1c, lipids in metabolic disease
- ACC/AHA Cardiovascular Prevention Guidelines — ahajournals.org — Lipids, blood pressure, cardiovascular risk
- ATA Guidelines for Thyroid Disease — thyroid.org — TSH, Free T4, thyroid antibodies
- NICE Clinical Guidelines — nice.org.uk — UK evidence-based clinical practice
- KDIGO Guidelines (Kidney) — kdigo.org — eGFR staging, creatinine interpretation
- Endocrine Society Clinical Practice Guidelines — endocrine.org — Hormones, adrenal, pituitary
- British Society for Haematology — b-s-h.org.uk — CBC, iron, haemoglobin guidelines
- WHO Haemoglobin Thresholds — who.int — Global anaemia reference
Accurate, non-commercial educational resources on blood tests and lab results.
- Lab Tests Online — labtestsonline.org — Published by the American Association for Clinical Chemistry (AACC); peer-reviewed patient education for every common test
- MedlinePlus Medical Tests — medlineplus.gov/lab-tests — NIH National Library of Medicine; authoritative US reference
- NHS A–Z Health Conditions — nhs.uk/conditions — UK National Health Service clinical information
- Mayo Clinic Lab Tests — mayoclinic.org — Patient-friendly test explanations with clinical context
- Merck Manual — merckmanuals.com — Professional clinical reference; free access
- The Menopause Society — menopause.org — Evidence-based perimenopause and menopause resources
- PubMed — National Library of Medicine; full access to biomedical literature
- Cochrane Reviews — Systematic reviews; highest evidence tier
- JAMA Network — American Medical Association journals
- BMJ — British Medical Journal; strong primary care and diagnostics coverage
- The Lancet — Clinical research across all specialties
- American Journal of Clinical Nutrition — Nutritional biomarkers and micronutrient deficiencies
This list is maintained as an open resource. Contributions welcome via pull request.
To add a resource:
- It must be publicly accessible (no paywalled content in the main links)
- Link to the primary source, not a summary or aggregator
- For clinical guidelines, link to the official publication, not a commentary
- No affiliate links, sponsored content, or pay-to-play listings
To suggest a correction: Open an issue with the incorrect information and a link to the authoritative source that contradicts it.
Not medical advice. Blood test interpretation requires clinical context and should involve a qualified healthcare provider. This guide is an educational resource.
