Medical Health Encyclopedia

Urine tests that are used to determine the specific chemical and biologic factors causing the stone should be performed about 6 six weeks after the attack, since the attack itself may change the levels of such substances, including calcium, phosphate, and citrate. It should be noted that calcium levels in the urine may be abnormal even in many people without stones. In addition, high urinary concentrations of calcium may pose a greater or lesser risk depending on age. (In one 2001 study, middle-aged adults with high urinary calcium concentrations had a much greater risk than older adults with high levels.)

Microscopic Examination

The kidney stones obtained from the sample are examined under a microscope. The crystal formations are often specific enough so that the doctor is able to identify the substance causing the stone.

• Calcium oxalate crystals are eight-sided (octahedral), while calcium phosphate crystals tend to have irregular shapes.
• Uric acid stones are sometimes described as pear- or diamond-shaped.
• Some struvite stones have very specific shapes commonly described as "coffin lids." Struvite crystals may also occur in a formation known as a staghorn, which can be large and damaging to the kidney.

Testing the Acidity of Urine

Testing whether urine is acid or alkaline helps to identify the specific type of stone. The levels of acid or alkaline in any solution, including urine, are indicated by the pH scale:

• A pH value of 7.0 is neutral.
• A solution with a low pH (below 7.0) is acidic. (A low pH favors uric acid and cystine stones.)
• A solution with a high pH is alkaline. (A high pH favors calcium phosphate and struvite stones.)

Testing for Blood in the Urine

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A dipstick for blood in the urine (called hematuria) is typically performed when patients appear in the emergency room with flank pain (the primary symptom of kidney stones). About a third of kidney stone patients, however, do not show blood in the urine, so other tests are needed.

Blood Tests