Blood Tests for Swollen Legs and Ankles
Swelling is a symptom, not a diagnosis. Here are the blood tests that find the real cause - and which kind of swelling never waits.
Understanding Why Legs Swell
Fluid normally sits inside blood vessels because the protein albumin holds it there, the kidneys clear excess, the heart pumps it onward, and the liver makes the protein in the first place. When ANY of those four organs falters, fluid leaks into the soft tissue - and gravity sends it to the lowest point: your feet and ankles.
That is why swollen legs are rarely a leg problem. They are usually a heart, kidney, liver or thyroid signal showing up where you can see it. A targeted blood panel can usually narrow down the cause in one visit.
One Leg vs Both Legs - The Critical Distinction
One leg swollen, warm, painful
Treat as deep vein thrombosis (DVT) until proven otherwise. Urgent ultrasound and D-dimer. A clot can travel to the lungs - this is not an “I will see if it settles” situation.
See a doctor today.
Both legs swollen, painless
Almost always systemic - heart, kidney, liver, thyroid, low albumin, or medication. Pace yourself, book a GP appointment, and ask for the full oedema panel.
Investigate this week.
8 Blood-Testable Causes of Leg Swelling
Heart Failure
Why this causes swelling: When the right side of the heart cannot keep up, blood backs up into the veins and fluid leaks into the tissue around the ankles. The swelling tends to be symmetric, pits when you press it, and is worst by evening.
How to recognise it: Breathlessness on mild exertion or lying flat, waking at night gasping, weight gain over days, and fatigue. Often pre-existing high blood pressure or a history of a heart attack.
Kidney Disease
Why this causes swelling: Failing kidneys cannot clear excess fluid and salt, and they leak protein (albumin) into the urine. Less albumin in the blood means water moves out of the vessels and into the legs - and around the eyes in the morning.
How to recognise it: Puffy face in the morning, foamy urine, fatigue, itchy skin, reduced or excessive urination. Diabetics and people with longstanding high blood pressure are at highest risk.
Liver Disease / Cirrhosis
Why this causes swelling: The liver makes albumin - the protein that keeps fluid inside blood vessels. In cirrhosis, albumin drops and pressure inside the portal vein rises. The combination causes fluid in the legs and ascites (fluid in the belly).
How to recognise it: Belly swelling, yellow tint to eyes or skin, easy bruising, spider veins on the chest, history of heavy alcohol use or hepatitis. Often the legs swell AFTER the abdomen.
Hypothyroidism (Myxoedema)
Why this causes swelling: Severe underactive thyroid causes a distinctive non-pitting swelling called myxoedema, where mucopolysaccharides build up in the tissue. Press the swelling and it does not dent like normal oedema.
How to recognise it: Cold intolerance, fatigue, weight gain, dry hair and skin, slow heart rate, hoarse voice. Often shows up first as puffy eyes and a thickened face.
Venous Insufficiency / Varicose Veins
Why this causes swelling: When the valves in the leg veins fail, blood pools in the lower leg. Pressure rises and fluid is pushed out into the tissue. Blood tests are usually NORMAL with chronic venous insufficiency - the diagnosis is mostly clinical and ultrasound-based.
How to recognise it: Aching legs that are worse standing and better elevating, visible varicose veins, brownish skin discolouration above the ankle, eczema-like skin changes, ulcers around the inner ankle.
Deep Vein Thrombosis (one-sided swelling)
Why this causes swelling: A blood clot in a deep leg vein causes one-sided swelling, warmth and pain. Risk factors include recent surgery, immobilisation, long flights, pregnancy, oral contraceptives, cancer, and inherited clotting disorders. DVT is a medical emergency because the clot can break free and lodge in the lungs.
How to recognise it: ONE leg swollen, warm, tender, and possibly red. Pain on flexing the foot upward. This is the pattern that must never be ignored. See a doctor today.
Low Albumin / Malnutrition
Why this causes swelling: When albumin in the blood is low for any reason (severe gut disease, malnutrition, malabsorption, sepsis), fluid leaks out of vessels. This is one of the most overlooked causes of generalised swelling.
How to recognise it: Soft pitting swelling across both legs, fatigue, weight loss, often a known illness (coeliac, Crohn s, anorexia, cancer). Easy to test and easy to correct once identified.
Medication Side Effect
Why this causes swelling: Calcium channel blockers (amlodipine, felodipine), some diabetes drugs (gliptins, pioglitazone), NSAIDs, steroids, and certain antidepressants all cause ankle swelling. Blood tests look for kidney or electrolyte changes from the medication.
How to recognise it: Swelling that appeared after starting a new medication, usually within weeks. Often resolves with a medication change.
The Pit Test - What It Tells You
Press firmly on your shin for 10 seconds, just above the ankle. Then let go. If you leave a dent that takes seconds to fill back in, that is “pitting oedema” - fluid in the tissue. The deeper and longer-lasting the dent, the more fluid. If the swelling does NOT pit, suspect myxoedema (hypothyroid), lymphoedema or fat tissue.
Take a quick photo with a ruler for scale before your GP visit. Pitting swelling is graded 1+ to 4+ depending on the dent depth. Your doctor will care about this detail.
What to Ask Your Doctor
Ready-to-use GP script:
“I have had swelling in [one leg / both legs] for [X days/weeks], mostly [after standing / in the evenings / constantly]. It [does / does not] pit when I press it. Could we check the oedema panel - kidneys, liver, albumin, BNP, thyroid, and electrolytes - so we can find out where this is coming from?”
Full Blood Count
Kidney function (eGFR, creatinine, urea)
Urine albumin:creatinine ratio (ACR)
Liver function (ALT, AST, GGT, bilirubin)
Albumin and total protein
TSH and Free T4
BNP or NT-proBNP
D-dimer (if DVT suspected)
Electrolytes (Na, K, Cl)
CRP / ESR
When Swelling Is NOT a Blood Problem
Not every swelling is a kidney or heart issue. These are common, harmless causes worth ruling in first:
Long flights and long sits
Hours in one position lets fluid pool. Resolves within a day of normal activity. If swelling lasts longer than 48 hours after a long-haul flight, get DVT excluded.
Hot weather and dependent oedema
Standing all day in the heat causes mild swelling in healthy people. Worse in late pregnancy.
Pregnancy
Some swelling is normal. Sudden swelling in face and hands plus headache or visual changes can mean pre-eclampsia - urgent assessment needed.
High-salt meal the day before
Soft, transient pitting on the lower legs that settles within 24-48 hours.
Track Your Heart, Kidney and Liver Markers
Upload your blood test results - SmarterBlood charts the markers that drive leg swelling so you can see whether something is creeping up over time.