Kidney stones are also called renal calculi, urolithiasis, and nephrolithiasis. Contrary to popular belief, they are mainly one of the four kinds: calcium oxalate, struvite, urate, and cystine stones. In this article, my purpose is to introduce these subtypes to you.
Calcium Oxalate/Calcium Phosphate Stones
The most common type is calcium oxalate and/or calcium phosphate that roughly account for 80% of all stones. As you can tell, the three components of these stones are calcium, oxalate, and phosphate. Calcium oxalate stones are by far more common than calcium phosphate stones. Calcium oxalate crystals are associated with high blood calcium (hypercalcemia), high urinary calcium (hypercalciuria), and excess vitamin C (ascorbic acid) intake. Calcium phosphate crystals are commonly associated with hyperparathyroidism. Note that the reason that vitamin C produces oxalic acid is that the liver converts it to oxalate. As a result, high doses of vitamin C causes high urinary levels of oxalate (oxaluria) and predisposes to calcium oxalate crystal formation.
The second most common type of kidney stones are struvite stones. These stones are also known as urease stones and infection-induced stones. They are by far the nastiest of all stones! Three components of these stones are magnesium, ammonium, and phosphate. They account for 10-15% of all kidney stones and are uniquely associated with alkaline urine. To make a useful generalization; “all kidney stones are formed in acidic urine with the exception of struvite stones.
Can you guess why do they call them urease- and infection-induced stones? Because they are associated with urinary tract infections due to urease-producing bacteria such as Proteus!
Uric acid (Gout) Stones
The third common type, 6% of all stones, are uric acid stones. They are associated with high urinary uric acid levels (hyperuricemia) and gout. It is noteworthy to mention that about 4% of US adults have gout. These stones in contrast to all others are translucent (transparent) and they do not show up on X-ray imaging. Also, about 25% of individuals who present with kidney stone and as a result reduced renal functioning, present with agonizing gout attacks.
Finally, the least common urinary stones are cystine stones that account for less than 1% of all stones. They are due to a genetic disease known as cystinuria. In contrast to other stones that tend to appear in individuals over 30 years of age, cystine stones typically appear between 10 and 30 years of age.
Please note that cysteine is the most insoluble amino acid, especially when two of them are joined together! In cystinuria kidneys cannot retain (reabsorb) cystine, and they are lost into urine and precipitated as cystine stones. Also note that when two molecules of cysteine that are more soluble, join together, they make cystine that is less soluble.