Blood Tests for Shortness of Breath
Breathlessness is rarely “just being unfit.” Here are the blood tests that reveal what is actually limiting your breath - and which ones cannot wait.
The 3 Big Categories of Breathlessness
Doctors think about breathlessness in three buckets: blood-related (not enough haemoglobin to carry oxygen), heart-related (not enough pump to circulate it), and lung-related (not enough air exchange). Blood tests are the fastest way to triage which of the three is the problem.
Blood
Anaemia, iron, B12, folate. Symptoms get worse on exertion, better with rest.
Heart
BNP, troponin. Worse lying flat, ankle swelling, waking gasping at night.
Lung
FBC, CRP, eosinophils. Wheeze, cough, sputum changes, smoking history.
8 Blood-Testable Causes
Anaemia / Low Haemoglobin
Why this causes breathlessness: Haemoglobin is the protein in red blood cells that ferries oxygen from your lungs to every tissue. When haemoglobin drops, your heart has to pump faster and your lungs have to work harder to deliver the same oxygen. The result is breathlessness on exertion - climbing stairs, walking uphill - that goes away with rest.
How to recognise it: Pale skin, brittle nails, hair loss, fatigue, cold hands and feet, palpitations, headaches. Iron deficiency is the most common cause in pre-menopausal women; B12 deficiency in older adults and vegans.
Heart Failure
Why this causes breathlessness: When the heart cannot pump strongly enough, fluid backs up into the lungs. Breathing feels like a struggle, especially lying flat or during mild activity. BNP is the blood marker released by the stretched heart - it is the single most useful test to distinguish heart vs lung causes of breathlessness.
How to recognise it: Swollen ankles, waking at night gasping, needing extra pillows to sleep, fatigue, weight gain over days from fluid. Often a known history of heart disease, hypertension or previous infarct.
Pulmonary Embolism
Why this causes breathlessness: A blood clot in the lungs blocks oxygen exchange. Onset is sudden - the patient was fine an hour ago. D-dimer is the key screening blood test, but a CT pulmonary angiogram is needed to confirm.
How to recognise it: Sudden breathlessness, sharp chest pain worse on breathing in, fast heart rate, sometimes blood-streaked sputum. Risk factors: recent surgery, long flight, oral contraceptive, cancer, immobility, leg swelling.
Thyroid Dysfunction
Why this causes breathlessness: Hyperthyroidism speeds the heart, increasing oxygen demand and causing exertional breathlessness. Severe hypothyroidism slows everything - including breathing muscle strength - and causes fatigue-style breathlessness.
How to recognise it: Hyper: weight loss, heat intolerance, palpitations, tremor, anxiety. Hypo: cold intolerance, weight gain, dry skin, fatigue, hoarse voice.
Asthma / COPD Exacerbation
Why this causes breathlessness: Blood tests do not diagnose asthma or COPD - lung function tests do. But eosinophils on a Full Blood Count flag allergic asthma, CRP shows infection-driven flares, and IgE confirms allergic patterns.
How to recognise it: Wheeze, cough, chest tightness, worse at night or with allergens, smoking history (for COPD). Often a known diagnosis with a recent worsening.
Infection / Pneumonia
Why this causes breathlessness: Lung infection inflames the airway and fills the alveoli with fluid, making oxygen exchange harder. White cell count rises, CRP rises sharply, and the patient feels unwell with fever and cough.
How to recognise it: Fever, productive cough, sharp chest pain on breathing, fatigue. Recent flu or COVID, frail elderly, or compromised immunity raises risk.
Kidney Failure / Fluid Overload
Why this causes breathlessness: When the kidneys cannot clear fluid, it accumulates in the lungs (pulmonary oedema) and causes breathlessness. The story is often progressive ankle swelling first, then breathlessness lying flat.
How to recognise it: Swollen ankles, fatigue, reduced or excessive urination, itchy skin, history of diabetes or hypertension.
Anxiety / Hyperventilation
Why this causes breathlessness: Anxiety causes real breathlessness through hyperventilation. The diagnosis is one of EXCLUSION - never accept “just anxiety” until anaemia, thyroid, cardiac, and lung causes are ruled out by blood tests and a chest x-ray.
How to recognise it: Sense of inability to take a deep breath, tingling around mouth or fingers, light-headedness, often during stress or panic episodes. Resolves with controlled breathing.
What to Tell Your Doctor
Doctors take breathlessness seriously. Walk in describing pattern, triggers and progression - they will order the right panel automatically.
What to say:
“I have been getting short of breath for [X weeks]. It happens when [I climb stairs / lie flat / at rest]. [It is / is not] getting worse. I [do / do not] have a cough, chest pain, or ankle swelling. Could we do an FBC, iron studies, BNP, thyroid and kidney function?”
Full Blood Count
Iron Studies (Ferritin, Iron, TIBC)
B12 and Folate
BNP or NT-proBNP
TSH and Free T4
Kidney function (eGFR, Creatinine)
CRP and ESR
D-dimer (if PE suspected)
Liver function tests
Electrolytes including Mg
Breathlessness Emergency Signs
See Your Oxygen-Carrying Capacity
Upload past results - SmarterBlood charts haemoglobin, iron and cardiac markers so you can see exactly when something started slipping.