Kidney stone analysis

Dr David Howdijam *

Kidney stones are small, hard masses that form within the kidneys. Kidney stone analysis uses one or more test methods to examine and determine the composition of a stone. This is done in order to help identify the cause of the stone and, where possible, to prevent the recurrence of more stones.

The kidney stones can be formed in the kidneys and cause problems either because they grow large enough to obstruct urine flow or because they become dislodged or break off and begin to travel from the kidney through the ureter; they can cause temporary obstruction and stretch, irritate and/or damage the walls of the ureters. This movement can cause abrupt, extremely severe pain that may be intermittent or continuous.

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 typically cannot pass from the kidney into the ureters, and for smaller stones that get into but do not pass through the ureters, some form of treatment is needed.

The stone may be surgically removed. With some stones, it is possible to use extracorporeal shock wave lithotripsy. This treatment pulverises the stone using targeted shock waves.

Stones can develop for several reasons, but the most common is because there is a high concentration of a particular chemical in the urine that precipitates and forms crystals. This can happen when a person produces and excretes an excess amount of the chemical.

Kidney stone analysis

A kidney stone analysis is performed to determine the chemical composition of a stone when it is filtered out of the urine or removed surgically from the urinary tract. A laboratory will also typically document the physical characteristics of a stone – its size, shape, colour, weight and texture. The stone may also be fractured so that its layers can be observed. One or more tests will be performed to determine the stone’s composition.

In addition to the stone analysis, blood and urine tests are often done to determine whether the individual produces excess chemicals that may contribute to the formation of kidney stones and to evaluate the person’s overall health. The test results can help distinguish between a probable stone and other conditions that may have similar symptoms but require different treatment.

Testing may include:

o Blood and 24-hour urine tests for calcium, uric acid, creatinine, and sometimes oxalate, citrate, phosphate, and/or cystine.
o Urinalysis – to detect red and white blood cells, crystals, signs of infection, and to measure urine pH.
o Complete Blood Count (CBC) – to evaluate white blood cells for signs of infection.

When is it done?

Kidney stone analysis is done when a person has passed kidney stone and it has been filtered out of the urine or when a stone has been removed from some part of the urinary tract through surgical procedure.

Signs and symptoms associated with a kidney stone may lead a health practitioner to search for a stone either in the urine passed out or within the body using imaging tests.

What does the test result mean?

A kidney stone analysis identifies the chemical composition of the stone. Common types of kidney stones include:
o Calcium oxalate
o Calcium phosphate
o Uric acid
o Struvite (magnesium and ammonium phosphate) – stones associated with bacterial infection
o Cystine – stones associated with an inherited excess of cystine excretion
o Drug-related – stones that are associated with drugs

A person with an underlying disease or condition may produce and/or excrete an excess of a specific chemical into the urine. Not drinking enough fluids and/or having urine with a high or low pH can contribute to a person’s risk of forming stones. Preventing kidney stones from developing again depends upon identifying and addressing the cause of the stone formation.

Some reasons why kidney stones are formed are:

o An inherited tendency to absorb more than the normal amount of calcium from the diet, causing high urine calcium.
o Dietary factors – consuming food or drink high in calcium or oxalate may result in increased amount of these substances in the urine.
o Factors affecting absorption of nutrients, such as inflammatory bowel disease or intestinal surgery – an imbalance in nutrient absorption may result in excess urinary calcium.
o A variety of metabolic disorders – these may lead to some degree of metabolic acidosis, resulting in an increased amount of calcium excreted in the urine.
o Uric acid stone may be formed when a person excretes excess uric acid due to conditions such as gout or disorders of uric acid metabolism.

Conclusion

Composition of kidney stone varies from patient to patient, based on the location where they are formed, food habits of individual and geographical location. As composition of urinary stones varies from place to place, qualitative analysis of crystals material present in kidney stones is essential to provide ideal treatment to patients with a history of recurrent stone formation.

A study of the chemical composition of kidney stones is important for understanding the aetiology as well and the therapy for stone disease is usually based on the analysis of calculi thus permitting proper management of the disease.