Anatomy
The kidneys are two bean-shaped organs located either side of the spine, just below the ribcage. Each contains around one million tiny filtering units called nephrons.
Organ Health · Kidney
Kidney Health
Understand how your kidneys filter waste, balance fluids and electrolytes, and support blood pressure — and which laboratory tests help assess kidney function.
Educational information only. Detectives Health helps explain laboratory tests and organ health in plain English. It does not diagnose disease, prescribe treatment or replace professional medical advice. Please discuss your individual results with a qualified healthcare professional.
Understanding the organ
Anatomy, function and importance
Function
The kidneys filter waste from the blood, regulate fluid and electrolyte balance, support blood pressure control, activate vitamin D and help produce red blood cells.
Why it matters
Healthy kidneys are essential for stable blood pressure, strong bones, balanced minerals and safe removal of medications and waste products.
How it works
A simple explanation in plain English
Blood enters each kidney and passes through microscopic filters that separate waste and excess fluid from useful substances.
Nutrients, electrolytes and most water are reabsorbed back into the bloodstream, while waste products are concentrated into urine.
The kidneys release hormones that support red blood cell production, activate vitamin D and help regulate blood pressure.
Laboratory tests
Which laboratory tests assess this organ
Each test contributes a small piece of information. Results are always interpreted alongside symptoms, medical history and other investigations.
Creatinine
- What it measures
- A waste product from normal muscle activity, cleared by the kidneys.
- Why it is requested
- A core marker of kidney function; used alongside age, sex and ethnicity to estimate filtration.
- Factors influencing results
- Muscle mass, hydration, protein intake and certain medications.
Urea
- What it measures
- A waste product from protein breakdown.
- Why it is requested
- Interpreted alongside creatinine to help assess hydration and kidney function.
eGFR
- What it measures
- An estimate of how much blood the kidneys filter each minute.
- Why it is requested
- Used to assess and monitor kidney function over time.
Sodium
- What it measures
- The main electrolyte in body fluids, important for fluid balance and nerve function.
- Why it is requested
- Reviewed to assess hydration, fluid balance and certain medication effects.
Potassium
- What it measures
- An electrolyte essential for heart and muscle function.
- Why it is requested
- Closely monitored because both high and low levels may affect the heart.
Chloride
- What it measures
- An electrolyte that works alongside sodium in fluid balance and acid–base regulation.
- Why it is requested
- Reviewed as part of an electrolyte panel.
Bicarbonate
- What it measures
- A marker of acid–base balance in the blood.
- Why it is requested
- Helps assess kidney contribution to acid–base regulation.
Calcium
- What it measures
- A mineral vital for bones, nerves and muscle function.
- Why it is requested
- The kidneys help regulate calcium alongside vitamin D and parathyroid hormone.
Phosphate
- What it measures
- A mineral that works closely with calcium in bone health.
- Why it is requested
- Often reviewed in kidney disease as phosphate handling changes with reduced filtration.
Factors that influence results
Why results vary between people and over time
Laboratory values are shaped by many day-to-day factors. Understanding them helps you and your healthcare professional interpret results in context.
Age
Kidney filtration naturally declines with age.
Hydration
Dehydration temporarily reduces filtration and may raise creatinine and urea.
Blood pressure
Uncontrolled high blood pressure is a leading cause of long-term kidney damage.
Diabetes
Long-term high blood sugar is a major cause of chronic kidney disease worldwide.
Medication
Some painkillers, blood pressure medicines and antibiotics may affect kidney tests.
Diet
Very high protein or high salt intake may influence kidney workload over time.
Muscle mass
People with more muscle naturally produce more creatinine.
Medical conditions
Heart failure, autoimmune conditions and infections may impact kidney function.
Preparing for your blood test
Practical educational guidance
Preparation varies between laboratory tests. Always follow the specific instructions from the clinician or laboratory that requested your sample.
- Follow any fasting instructions given by the requesting clinician — some tests such as fasting glucose or a lipid profile need 8–12 hours without food.
- Take regular medication as usual unless your healthcare professional advises otherwise.
- Mention any supplements — biotin, high-dose vitamins and herbal remedies can influence several laboratory tests.
- Stay well hydrated with water in the hours before your test; dehydration can affect several markers.
- Avoid unusually strenuous exercise in the 24 hours before testing, as this may temporarily alter some results.
- Where possible, attend at a consistent time of day — several hormones and enzymes follow daily rhythms.
- Postpone routine testing if you have an acute infection, unless your clinician specifically wants a result during illness.
Healthy lifestyle
Everyday habits that support this organ
Consistent healthy habits may support organ function over time. They do not replace medical assessment when concerns arise.
Stay hydrated
Adequate water intake supports filtration and helps prevent kidney stones.
Control blood pressure
Keeping blood pressure within target ranges is one of the most protective steps for kidney health.
Manage blood sugar
Good glucose control significantly reduces the risk of diabetic kidney disease.
Balanced diet
A diet rich in vegetables, fruits and whole grains supports kidney and cardiovascular health.
Reduce salt
Lower salt intake helps control blood pressure and reduces kidney strain.
Regular movement
Exercise supports blood pressure, weight and metabolic health.
Avoid smoking
Smoking damages blood vessels including those supplying the kidneys.
Careful painkiller use
Long-term regular use of certain painkillers may affect the kidneys — discuss with your pharmacist.
Symptoms to discuss
Changes worth mentioning to a healthcare professional
These symptoms do not confirm any diagnosis. They are educational prompts for a professional assessment when they are new, persistent or worsening.
- • Persistent tiredness with poor concentration
- • Swelling of ankles, feet or around the eyes
- • Foamy or bloody urine
- • Changes in how often you pass urine
- • Loss of appetite or metallic taste
- • Persistent itching
- • Muscle cramps
- • Difficult-to-control blood pressure
Seek urgent medical assessment
In the UK call 999 for emergencies, or 111 for urgent advice. If you experience any of the following, seek help without delay:
- • Little or no urine output for many hours
- • Severe swelling with shortness of breath
- • Sudden confusion with known kidney disease
- • Severe pain in the back or side with fever
Routine monitoring
Health checks and screening in an educational context
Recommendations vary between individuals. Your healthcare professional will advise which tests apply to you and how often.
- Include kidney function tests in routine health reviews if advised
- Attend annual kidney checks if you have diabetes or high blood pressure
- Monitor blood pressure regularly
- Review medications for kidney impact at pharmacy or GP reviews
- Discuss family history of kidney disease with your healthcare professional
- Follow personalised monitoring plans for known kidney conditions
Questions you may wish to discuss
Prompts for your next healthcare appointment
These are educational conversation starters — not a script. Bring the ones that feel most relevant to your situation.
- What does this result mean in my individual situation?
- Should the test be repeated, and if so when?
- Are further investigations recommended based on this result?
- Could any of my medications or supplements be influencing the result?
- Are lifestyle changes likely to help, and which ones would you prioritise?
- How does this result fit with my symptoms and medical history?
- What would prompt a change of plan or a specialist referral?
Frequently asked questions
Common educational questions
Q.What is chronic kidney disease?
Chronic kidney disease describes long-term reduced kidney function. It is often silent in the early stages and picked up through routine blood or urine tests.
Q.Do painkillers harm the kidneys?
Occasional use of over-the-counter painkillers is usually safe. Long-term regular use of some anti-inflammatories may affect the kidneys and should be reviewed.
Q.How much water should I drink?
There is no single answer — aim for pale straw-coloured urine as a general guide. Individual needs vary with activity, climate and medical conditions.
Q.Can kidney function improve?
In some situations kidney function can recover, particularly if the cause is reversible. Ongoing damage may be slowed with good blood pressure and diabetes control.
Biomedical Scientist's insight
The science behind your result
Biomedical Scientists perform, validate and quality-check laboratory analyses before results are authorised for release. Every test is run against calibrated standards and internal controls, and reviewed for analytical accuracy. Laboratory findings are then interpreted by healthcare professionals alongside your symptoms, examination findings and medical history — which is why context matters as much as the number on the report.
Continue learning
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Scientific leadership
Steve Diongo
Founder & Scientific Lead · HCPC-Registered Specialist Biomedical Scientist · 15+ years NHS · 20+ years laboratory medicine
This module has been developed and reviewed by the Detectives Health Professional Team under the scientific leadership of Steve Diongo. Educational content is regularly updated using recognised laboratory standards, current scientific evidence and professional best practice.
Reviewed against NHS, NICE, CDC, WHO and USPSTF guidance.
Educational information only. Detectives Health does not diagnose disease, prescribe treatment or replace professional medical advice.
Please discuss your individual laboratory results, symptoms and health concerns with a qualified healthcare professional.