Stone analysis

Kidney stone analysis involves testing the stone(s) to determine its composition. This helps to identify the cause of the stone and may help to prevent the formation of more stones.

The urinary tract consists of two kidneys, two ureters, the bladder, and the urethra. The kidneys filter the blood to remove waste substances, which are excreted from the body in the urine. Urine is transported from the kidneys to the bladder through tube-like ureters and eliminated from the bladder through the urethra. This is a continual process of waste filtration, urine production, and elimination.

Kidney stones, also known as renal stones, urinary tract stones or calculi, can form in the kidneys and in other parts of the urinary tract. Kidney stones can cause problems either because they grow large enough to obstruct urine flow or because they become dislodged and begin to travel from a kidney along the ureter. They can cause temporary obstruction and stretch, irritate, and damage the walls of the ureters. This movement often causes sudden, extremely severe pain that may be intermittent or continuous (‘renal colic’).

Many stones will eventually pass out of the body in the urine, but some are too large or have too irregular a shape for the body to expel. With very large stones, which cannot pass from the kidney into the ureters, and for smaller stones that get stuck in the ureters, some form of treatment is needed. The stone may need to be surgically removed, often using devices that pass through the urethra and bladder to the site of the stone (cystoscope or ureteroscope). With some stones, it is possible to use extracorporeal shock wave lithotripsy. A beam of ultrasound shock waves from outside the body is focussed onto the stone, which breaks into smaller pieces. The smaller fragments that remain can then pass through the urinary tract.

Stones can develop for many reasons, but the most common is because there is a high concentration of a particular chemical substance in the urine that precipitates and forms crystals. This can happen if the urine is concentrated throughout the day because of a low fluid intake. It can also happen when a person produces and excretes an excess amount of the substance. Depending on how much and what type of material crystallises and where it forms, a kidney stone may be round, sharp and pointy or irregular with branches (called a staghorn). It can range in size from a grain of sand to bigger than a golf ball.

The composition of the stone depends upon the chemicals present in excess. It may be all one chemical compound or have different compounds in different layers. Common types of kidney stones include :

• Calcium oxalate
• Calcium phosphate
• Uric acid
• Struvite (magnesium ammonium phosphate) - stones associated with a bacterial infection

These four types (or mixtures of these) make up about 95% to 99% of kidney stones. About 75% of stones will contain calcium. Less common stones include:

• Cystine - stones produced because of an inherited condition that results in loss of cystine into the urine
• Drug-related - stones that are caused by crystallisation of drugs such as guaifenesin, indinavir, triamterene, atazanavir, and sulfonamides.

In the UK, about one in ten people will have a urinary stone sometime in their lives. Men are more likely to have stones than women. People who have had a stone in the past are more likely to develop stones in the future.

How is the sample collected for testing?

Most stones are collected during a surgical operation, in which case the hospital staff will send any material for testing.

If an operation is not required, you may be asked to pass urine through a small sieve for a few days to catch any stones that pass naturally. This is necessary because there is no way to determine exactly when a stone will pass out of the body. Bearing in mind that stones may be as small as grains of sand, any particles or obvious stones should be placed into a clean container provided by the doctor and then returned to the laboratory. It is important not to add anything to the stone, such as tissue or tape, as this can make testing more difficult.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.