Types of Kidney Stones: Implications for Diagnosis and Treatment

Kidney Stones.

The majority of kidney stones are made of calcium (75%).  However, there are a variety of kidney stones made of calcium and other components that have different etiologies and treatments.  For some stones (i.e. cystine stones), knowing the stone composition provides valuable information to diagnose the cause and initiate treatment of the causative condition.  Since most stones contain calcium, knowing the type of stone and the underlying cause help the practitioner create a rational treatment plan for a given patient.  Read more about the variety of kidney stones in this blog:

Calcium Containing Kidney Stones

As stated above, approximately 75% of kidney stones contain calcium.  The most common calcium containing stones include: calcium oxalate (60%), followed by hydroxyapatite (20%) and the rare, brushite (2%) stones.  The most common metabolic abnormalities associated with calcium-containing kidney stones include:
containing stone is

• hypercalciuria (most common abnormality)
• hypocitraturia
• hyperuricosuria
• hyperoxaluria

In general, these metabolic abnormalities lead to supersaturation of the urine with calcium.  Supersaturation refers to the condition when more solute (i.e. calcium) is in a fluid (i.e. urine) that can be chemically dissolved.  The above conditions either increase the amount of calcium in the urine, alter the pH (acidic content of the urine) or modify the likelihood of stones to form.  

Electron microscopy of calcium
oxalate crystals.

Calcium Oxalate Stones

Both calcium and oxalate are naturally occuring chemicals in the body and the urine.  Therefore, calcium oxalate stones can form with a variety of conditions that lead to increased calcium in the urine (hypercalciuria) and oxalate (naturally found in vegetables, fruits and nuts).  One common factor is that calcium oxalate stones often form when the urine is alkaline (pH>5.5).  

The needle-like crystals of calcium
phosphate in the urine.

Calcium Phosphate Stones

Like calcium oxalate stones, calcium phosphate stones are made of normally occurring components of the urine when the urine pH is alkaline (>7.0).  Calcium phosphate stones are also known as apatite or brushite stones.  Calcium phosphate stones form needle-like crystals in the urine (below).

Non-Calcium Containing Stones

Uric acid crystals.

Uric Acid Stones

Uric Acid (7%) stones form only in acid urine.  People more likely to form uric acid stones have conditions that predispose to acidic urine including: low urine output, a diet high in animal protein (such as red meat), high alcohol intake, and may have obesity, gout or inflammatory bowel disease.

Struvite Stones

The coffin-lid appearance of struvite
stones in the urine.

Struvite (7%) stones are made of ammonium magnesium phosphate.  They have a typical coffin-lid appearance on microscopy. These stones occur in alkaline-urine only, alkaline urine is produced by urease-producing bacteria, therefore these are otherwise known as "infectious stones."  These stones are common in patients with recurring urinary tract infections, especially those with spinal cord injuries and neurogenic bladder, as these patients are predisposed both to UTI and bone metabolism disorders in which calcium is mobilized.

Cystine Stones

Cystine kidney stones.

Cystine (1-3%) stones form in patients with impaired resorption of cystine.  For most people, these stones form as a result of a genetic condition, called cystinuria, that runs in families.  Therefore, the average cystine stone patient presents at a younger age with multiple stones.

Rare Kidney Stones

There are a number of extremely rare kidney stones (<1%) that are formed in uncommon genetic conditions or by a variety of medications.  These include xanthine, matrix stones and stones made of triamterene, silica, and 2,8-dihyroxyadenine.