Kidney Failure and Detection in Urine

Spec Mapping — OCR H420 Module 5.1.2 — Excretion, content statements covering the causes and consequences of kidney failure, the principles and comparison of haemodialysis, peritoneal dialysis and renal transplantation, and the use of urine analysis (including monoclonal-antibody-based pregnancy tests) as a diagnostic tool (refer to the official OCR H420 specification document for exact wording).

When the kidneys fail, the body rapidly loses its ability to regulate water, solute concentrations, blood pH, and nitrogenous waste. Without intervention, kidney failure is fatal within days. Modern medicine offers several life-saving options — haemodialysis, peritoneal dialysis and transplantation — each with its own advantages and limitations. Urine is also used as a diagnostic tool: its composition reveals information about kidney function, metabolism, pregnancy, drug use and disease. This final lesson covers causes and treatment of kidney failure, and common uses of urine analysis, matching OCR A-Level Biology A specification module 5.1.2(i)–(j).

The diagnostic use of urine is one of the oldest practices in medicine — uroscopy ("water-gazing") was used by ancient Greek and medieval physicians long before the underlying physiology was understood. Modern testing combines simple colour-change chemistry (Benedict's reagent for reducing sugars, dipstick enzymatic glucose tests) with sophisticated immunoassays based on monoclonal antibodies — antibodies produced by a single clone of B cells, each binding a single epitope with high specificity. The technique of producing monoclonal antibodies was developed by Georges Köhler and César Milstein at Cambridge in 1975 (work that earned them shares of the 1984 Nobel Prize). The home pregnancy test is one of the great success stories of monoclonal-antibody biotechnology; the broader principles transfer directly to immunoassays for hormones, drugs, and disease biomarkers across modern clinical biochemistry. The historical work is paraphrased here; original publications are not quoted verbatim.

Key Definitions:

• Kidney failure — the loss of kidney function, classified as acute (sudden) or chronic (progressive).
• Dialysis — a procedure that filters blood artificially, removing wastes and excess water when the kidneys cannot.
• Haemodialysis — dialysis in which blood is passed through an external machine.
• Peritoneal dialysis — dialysis in which the peritoneal membrane in the abdomen is used as the dialysis surface.
• Monoclonal antibody — an antibody produced by a single clone of B-cells, specific for a single epitope; used in diagnostic tests.

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Causes of Kidney Failure

Kidney failure can be acute (developing over hours or days) or chronic (developing over months or years). OCR expects you to know several main causes:

Diabetes Mellitus

Diabetes — particularly type 2, which is becoming more prevalent — is the leading cause of chronic kidney failure worldwide. Persistently high blood glucose damages the glomerular capillaries and the basement membrane over many years. This is called diabetic nephropathy and begins with leakage of protein (albumin) into the urine, progressing to reduced GFR and ultimately complete failure.

Hypertension (High Blood Pressure)

Chronic high blood pressure damages the small vessels of the glomeruli. Thickening and narrowing of these vessels reduces filtration and eventually destroys nephrons. Hypertension and diabetes together account for the majority of cases of chronic kidney disease.

Infections

• Pyelonephritis — bacterial infection of the kidney, often spreading up from the bladder (especially in women), causing inflammation and scarring of the nephrons.
• Glomerulonephritis — inflammation of the glomeruli, often triggered by an immune response after infection with certain streptococci. Immune complexes deposit in the basement membrane, damaging the filtration barrier.

Other Causes

• Obstruction of urine flow — kidney stones, prostate enlargement, tumours. Prolonged obstruction damages the kidney by back-pressure.
• Trauma or haemorrhage — a sharp fall in blood pressure reduces blood flow to the kidneys, causing acute tubular necrosis.
• Toxins — heavy metals, some drugs (especially NSAIDs and certain antibiotics in overdose), and contrast dyes used in imaging.
• Genetic diseases — polycystic kidney disease, in which cysts replace functional kidney tissue.

The Consequences

As kidney function declines, the following problems develop:

• Build-up of urea and creatinine (uraemia), leading to nausea, fatigue and mental confusion.
• Fluid overload — fluid accumulates in lungs (pulmonary oedema) and legs (peripheral oedema).
• Acidosis — inability to excrete H⁺ drops blood pH.
• Hyperkalaemia — high blood K⁺ causes dangerous cardiac arrhythmias.
• Anaemia — reduced production of erythropoietin, the hormone that stimulates red blood cell production.
• Bone disease — reduced activation of vitamin D affects calcium handling.

Below about 10 % of normal function, dialysis or transplantation is required for survival.

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Treatment of Kidney Failure

Haemodialysis

In haemodialysis, blood is removed from the patient (usually via an arteriovenous fistula in the arm), passed through a dialyser, and returned. Inside the dialyser, blood flows through many fine tubes made of a partially permeable membrane, while dialysis fluid flows over the outside in the opposite direction — a countercurrent exchange.

• The dialysis fluid contains normal concentrations of glucose, Na⁺, K⁺ and other useful ions. It contains no urea and no creatinine.
• Waste products (urea, creatinine) therefore diffuse from the blood into the dialysis fluid down their concentration gradient.
• Excess water is removed by applying a positive pressure to the blood compartment.
• Useful substances do not diffuse out because they are already at equal concentration on both sides.

A haemodialysis session typically lasts 3–5 hours and must be carried out three times a week. Patients must restrict their fluid and protein intake between sessions to prevent overload. Life expectancy on dialysis is much shorter than with a functioning transplant.

flowchart LR
    A["Patient’s blood<br/>high urea"] --> B[Dialyser]
    B --> C["Cleaned blood<br/>low urea"]
    C --> A
    D["Dialysis fluid<br/>no urea"] --> B
    B --> E["Waste fluid<br/>with urea"]

Peritoneal Dialysis

In peritoneal dialysis, the patient's peritoneum (the thin, richly vascularised membrane lining the abdominal cavity) is used as the dialysis membrane. A permanent catheter is inserted through the abdominal wall. Sterile dialysis fluid is run into the peritoneal cavity, where it is left for several hours. Waste products diffuse from the peritoneal capillaries into the fluid across the membrane. The fluid is then drained out and replaced with fresh fluid.

• Can be done at home by the patient.
• Less disruptive than haemodialysis — often carried out overnight with a machine that cycles fluid automatically.
• Risk of infection (peritonitis) is the main complication.
• Less effective than haemodialysis for patients with very high protein catabolism.

Kidney Transplantation

The gold standard treatment. A healthy kidney from a donor (living relative or deceased organ donor) is implanted in the lower abdomen, with its artery, vein and ureter connected to the recipient's vessels and bladder.

Advantages:

• Restores near-normal kidney function.
• Freedom from the time commitment of dialysis.
• Better long-term survival and quality of life.

Disadvantages:

• Shortage of donor organs; long waiting lists.
• Requires lifelong immunosuppressive drugs to prevent rejection, which increase infection and cancer risk.
• Major surgery with its own risks.
• Rejection can still occur, eventually requiring a return to dialysis.

Ethical considerations: opt-in vs opt-out organ donation schemes; living donors; the risk to a relative of donating a kidney; the equitable allocation of scarce organs.

Comparison

Feature	Haemodialysis	Peritoneal dialysis	Transplant
Location	Hospital or centre	Home	Hospital (surgery), then outpatient
Frequency	3 sessions per week	Continuous	One-off (with follow-up)
Main risk	Clotting, vascular access problems	Peritonitis	Rejection, drug side-effects
Lifestyle impact	High	Moderate	Low (after recovery)
Cost	High (recurring)	Moderate	High (upfront), lower long-term
Long-term survival	Shorter	Shorter	Longer

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Detecting Substances in Urine

Urine has long been used as a diagnostic fluid because it contains the products of filtration and metabolism in convenient form. Modern tests rely on simple colour-change chemistry or on highly specific monoclonal antibodies.

Glucose in Urine

• Normally: no glucose should be detectable. The PCT reabsorbs ~100 % of filtered glucose via Na⁺-coupled cotransporters.
• In untreated diabetes mellitus, blood glucose rises so high that the filtered load exceeds the tubular maximum (Tm) for glucose. Excess glucose passes into the urine — glycosuria.
• Historical tests used Benedict's reagent; modern test strips use the enzyme glucose oxidase coupled to a colour indicator.

Protein in Urine

• Normally: very little protein. The basement membrane blocks albumin.
• In diabetic nephropathy, glomerulonephritis and hypertension, damage to the filtration barrier allows albumin through — proteinuria is an early warning of kidney disease.

Pregnancy Testing

Pregnancy tests detect human chorionic gonadotrophin (hCG), a hormone produced by the developing placenta shortly after implantation. hCG is small enough to be filtered at the glomerulus and to appear in the urine of pregnant women.

A modern home pregnancy test is an immunoassay using monoclonal antibodies:

1. Urine is applied to one end of the test strip.
2. It encounters mobile anti-hCG monoclonal antibodies linked to coloured dye particles. If hCG is present, it binds to these antibodies, forming a dye-labelled complex.
3. The liquid wicks along the strip and passes a test line — a region of immobilised antibodies that bind hCG at a different site. These trap any hCG-dye complex, creating a visible coloured line.
4. Excess unbound antibody continues along the strip to a control line, which contains antibodies that bind the mobile antibodies regardless of whether hCG is present. The control line should always show a colour, confirming the test has worked.

Two lines = positive. One line = negative. No lines = test failed.

flowchart LR
    A[Urine sample] --> B["Mobile anti-hCG antibodies<br/>dye-labelled"]
    B --> C{hCG present?}
    C -->|Yes| D["Forms dye complex<br/>trapped at test line"]
    C -->|No| E[No test line]
    D --> F["Control line:<br/>excess antibody trapped"]
    E --> F

Modern tests can detect hCG concentrations as low as ~20 IU L⁻¹, allowing pregnancy to be detected within days of implantation.

Anabolic Steroids

Anabolic steroids are synthetic testosterone derivatives sometimes used illegally by athletes to build muscle and speed recovery. They are metabolised by the liver and their metabolites excreted in urine, where they can be detected by:

• Gas chromatography / mass spectrometry (GC-MS) — the gold standard used by anti-doping agencies. Urine is vaporised and the molecules separated by gas chromatography, then identified by their characteristic mass spectrum.
• Immunoassays — use monoclonal antibodies specific to known metabolites for faster screening, followed by GC-MS confirmation of positives.

Modern techniques can detect steroids weeks or even months after they were taken, because metabolites persist in body tissues.

Other Substances Detected in Urine

Substance	What it indicates
Glucose	Diabetes mellitus (exceeds Tm)
Protein (albumin)	Diabetic nephropathy, hypertension, glomerulonephritis
Ketones	Diabetic ketoacidosis, prolonged fasting
Blood	Infection, kidney stones, tumour
Bilirubin	Liver disease or blocked bile duct
Nitrites	Bacterial urinary tract infection
hCG	Pregnancy
Drugs and their metabolites	Substance use / abuse testing

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Exam Tip

In questions about home pregnancy tests, always mention monoclonal antibodies and two sites: one mobile (dye-linked) antibody that captures hCG, and one immobilised antibody on the test line that binds hCG at a different epitope. Explaining the control line earns an additional mark — it uses antibodies against the antibody to confirm the test has run properly.

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Common Exam Mistakes

• Describing dialysis as "pumping out" waste. Wastes move by diffusion across a partially permeable membrane down their concentration gradient.
• Saying dialysis fluid contains "nothing". It contains normal concentrations of glucose, ions and salts, so these do not diffuse out of the blood.
• Forgetting the countercurrent flow in the dialyser. It maintains a concentration gradient along the whole length of the exchange surface.
• Confusing rejection with infection. Rejection is immune attack on the transplant; immunosuppressants reduce rejection but increase infection risk.
• Saying hCG is made by the mother. It is made by the placenta (of embryonic origin).
• Forgetting the control line on pregnancy tests. It is essential for quality assurance.
• Describing monoclonal antibody production as "random". They come from a single clone of B cells, fused with myeloma cells to make hybridomas — all identical.
• Listing glucose in urine as "normal". It is not present in healthy urine; its presence is a sign of disease.

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Quick Recap

• Chronic kidney failure is most often caused by diabetes and hypertension; other causes include infection, obstruction, trauma and genetic disease.
• Consequences include uraemia, fluid overload, acidosis, hyperkalaemia, anaemia and bone disease.
• Treatment options are haemodialysis, peritoneal dialysis and transplantation. Each has advantages and limitations.
• Dialysis relies on diffusion across a partially permeable membrane; dialysis fluid has normal concentrations of useful substances and none of the wastes.
• Urine analysis is a powerful diagnostic tool. Glucose indicates diabetes, protein indicates kidney damage, hCG indicates pregnancy.
• Pregnancy tests use monoclonal antibodies in a lateral flow device with mobile (dye-linked) antibodies, an immobilised test line and a control line.
• Anabolic steroids are detected in urine by GC-MS, the gold standard of anti-doping testing.

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SVG — Schematic of a Haemodialyser (Counter-Current Exchange)