Kidney Stones - Diagnosis

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.

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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

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

Blood Tests for Stone Factors. Blood tests may help determine levels of blood urea nitrogen, creatinine, calcium, phosphate, and uric acid for patients with known or suspected calcium oxalate stones. These tests are often scheduled about six weeks after the attack, particularly with recurrent stones, in order to measure these substances when the stone has been passed and the patient has been stabilized.

Parathyroid Tests. Tests to detect parathyroid hormone levels are administered if the doctor suspects hyperparathyroidism based on other signs and symptoms.

Tests for Infection. A test result that shows a high white blood cell count might indicate infection, but such results could be misleading, since white cells could also increase in response to the extreme physical stress of a kidney stone attack.