Urine in medical diagnosis (7.2.8) | OCR A-Level Biology Notes

OCR Specification focus:
‘Urine tests can diagnose conditions, including pregnancy testing using monoclonal antibodies, and testing for anabolic steroids and drugs.’

Urine analysis provides valuable diagnostic information because metabolic waste products and hormones are excreted in urine. Medical urine testing identifies physiological or pathological changes through chemical and immunological detection.

Composition of Urine and its Diagnostic Value

Urine is a filtrate of blood plasma containing water, urea, ions, and other metabolic by-products. Normally, it is free from cells, proteins, and large molecules. Abnormal components may indicate disease or metabolic dysfunction.

Normal Constituents of Urine

Water – makes up approximately 95% of urine.
Urea – a nitrogenous waste produced from amino acid deamination in the liver.
Ions – including sodium, potassium, chloride, and phosphate, regulated by kidney function.
Creatinine – produced from creatine phosphate breakdown in muscles.
Small organic molecules – such as uric acid and ammonia.

Abnormal Urine Constituents

The presence of unusual substances in urine often reflects underlying medical issues:

Glucose – may indicate diabetes mellitus, due to inability to reabsorb glucose in proximal tubules.
Protein (proteinuria) – suggests kidney damage or increased glomerular permeability.
Blood (haematuria) – could indicate infection, kidney stones, or tumours.
Ketones – suggest fat metabolism in uncontrolled diabetes or starvation.

Principles of Urine Testing

Urine testing is based on biochemical and immunological techniques that detect specific molecules or metabolites.

Biochemical Tests

Chemical strips, often called dipsticks, contain reagents that react with target molecules:

Glucose oxidase test for glucose.
Protein indicators change colour with albumin.
pH indicators measure acidity.
Nitrite tests detect bacterial infections.

Results are interpreted through colour comparison with a reference chart.

Pregnancy Testing and Monoclonal Antibodies

Pregnancy tests are a key example of diagnostic urine testing that utilises immunoassay techniques involving monoclonal antibodies.

The Hormone Detected: hCG

Human chorionic gonadotrophin (hCG) is secreted by the developing embryo soon after fertilisation and excreted in urine.

Monoclonal antibody: Identical antibodies produced from a single clone of B lymphocytes, all recognising the same specific antigen.

Mechanism of a Pregnancy Test

Pregnancy tests use lateral flow immunoassay technology to detect hCG.
Steps include:

Application zone – Urine sample containing hCG binds to mobile monoclonal antibodies with attached coloured beads.
Test line – Complexes move along the strip by capillary action and encounter immobilised monoclonal antibodies that capture hCG, forming a visible coloured line.
Control line – A second line forms to confirm the test is functioning correctly by binding excess mobile antibodies.

A visible test line indicates hCG presence, confirming pregnancy. The control line ensures validity.

Diagram of a urine lateral-flow immunoassay pregnancy test. hCG in the sample binds labelled monoclonal antibodies, then is captured at the test line; excess label binds at the control line to verify function. The figure focuses on the sandwich immunoassay format used in home pregnancy tests. Source.

Testing for Anabolic Steroids

Athletes and patients may be tested for anabolic steroids, which are synthetic derivatives of testosterone used to promote muscle growth. Their detection in urine is crucial for medical and ethical monitoring.

Principle of Steroid Testing

Anabolic steroids are lipid-soluble molecules that are metabolised slowly and excreted in urine. Detection involves:

Gas chromatography (GC) to separate compounds based on volatility and mass.
Mass spectrometry (MS) to identify molecules by analysing their unique mass-to-charge ratio.

Together, these methods (GC-MS) provide highly specific and sensitive identification of prohibited substances.

Block diagram of GC–MS used in confirmatory urine analysis. Compounds are separated by GC, ionised and fragmented in the MS ion source, then separated by m/z in the analyser and detected as a characteristic spectrum. This schematic captures the analytical logic without extraneous instrument detail. Source.

Gas chromatography (GC): Technique that separates components of a mixture by their different volatilities as they pass through a column with a carrier gas.

Mass spectrometry (MS): Analytical technique that identifies substances by measuring the mass-to-charge ratio of ionised particles.

The combination of GC and MS ensures accurate confirmation of steroid use, distinguishing between natural hormones and synthetic analogues.

Testing for Recreational Drugs

Urine testing also identifies recreational drug use, detecting compounds or their metabolites. These include cannabis, cocaine, opiates, and amphetamines.

Stages of Drug Testing

Screening – Rapid immunoassay tests detect target substances using antibodies specific to drug molecules.
Confirmation – Positive samples are re-analysed by GC-MS to verify accuracy and avoid false positives.

Photograph of a multi-drug urine test device used for screening via lateral-flow immunoassays. Individual panels target common drug classes, producing line-based positive or negative readouts. Extra detail beyond the OCR spec is limited to the multi-panel format typical of clinical screening. Source.

Advantages of Urine Drug Testing

Non-invasive collection.
Long detection window as many drugs remain in urine for several days.
Reliable quantitative analysis when using confirmatory testing.

However, urine drug testing reflects past exposure, not current intoxication levels.

Quality Control and Reliability

For clinical and diagnostic reliability, testing must be standardised and controlled:

Calibration of testing equipment ensures consistency.
Controls (positive and negative) verify accuracy.
Chain of custody protocols maintain sample integrity in forensic contexts.
Cut-off concentrations prevent false results due to passive exposure.

Proper interpretation considers metabolism rate, hydration, and time since exposure.

Broader Medical Uses of Urine Analysis

Beyond pregnancy and drug testing, urine diagnostics are expanding due to advances in biotechnology:

Detection of hormone imbalances (e.g., cortisol for stress assessment).
Monitoring of metabolic disorders (e.g., phenylketonuria).
Identification of renal dysfunction biomarkers.
Screening for infection through bacterial antigen detection.

These developments underline urine’s diagnostic versatility and its importance in preventive medicine and public health monitoring.

Ethical and Practical Considerations

While urine testing provides valuable health data, it raises ethical issues:

Consent and privacy in testing, particularly in workplaces or sports.
Potential for discrimination based on test results.
Accuracy and false positives, which may have significant consequences.

Healthcare professionals must ensure testing is conducted ethically and results are interpreted in context.

Urine diagnostics exemplify how excretory products reveal internal physiological states. By applying biochemical and immunological principles, modern medicine utilises urine to detect hormones, drugs, and metabolites, contributing to early diagnosis and effective health management.

FAQ

False positives can occur when substances other than hCG interact with the antibodies in the test.

Certain fertility treatments contain hCG, leading to a positive result despite no pregnancy.
Some tumours, such as trophoblastic or germ cell tumours, may produce hCG.
Improper test timing or reading the result too late can also cause misleading results due to evaporation lines forming on the test strip.

Proper sample collection and following manufacturer instructions minimise the chance of error.

Anabolic steroids are lipid-soluble and pass through the liver, where they undergo oxidation, reduction, and conjugation reactions to become more water-soluble.

These metabolites are then transported via the bloodstream to the kidneys, where they are filtered and excreted in urine.

Because metabolism rates vary between individuals, detection windows differ—some steroids remain traceable for weeks or months after use.

Urine testing is preferred because:

It is non-invasive and easy to collect in sufficient volume.
Drug metabolites accumulate in urine at higher concentrations than in blood.
The detection window is longer since many substances remain in urine after being cleared from the bloodstream.

Blood testing is used when recent use must be confirmed, while urine provides a record of past exposure.

A mouse is injected with the target antigen (e.g. hCG).
The mouse produces B lymphocytes that make specific antibodies.
These cells are fused with myeloma cells to create hybridomas, which can divide indefinitely.
Each hybridoma produces identical monoclonal antibodies, which are harvested and purified for use in test kits.

This ensures every test contains a consistent and specific antibody supply, improving reliability.

Urine drug testing raises concerns about:

Privacy and consent, particularly in workplaces or schools where individuals may feel coerced.
False positives leading to unfair disciplinary action or stigma.
Data confidentiality, as results reveal personal health information.

Ethical practice requires informed consent, secure handling of results, and ensuring tests are used solely for their intended purpose.

Practice Questions

Question 1 (2 marks)
Explain how monoclonal antibodies are used in pregnancy testing.

Mark Scheme

1 mark: States that monoclonal antibodies bind specifically to the hormone hCG found in the urine of pregnant women.
1 mark: Describes that this binding produces a visible coloured line (test line) on the pregnancy test strip, indicating the presence of hCG.

Question 2 (5 marks)
Describe how urine samples are used in the medical diagnosis of anabolic steroid or drug use. Include reference to the types of tests and how accuracy is ensured.