Kidney stones are small, hard mineral deposits that form inside the kidneys, the body’s primary organs for filtering waste from the blood. They are created when high concentrations of substances like calcium, oxalate, and uric acid crystallize and bind together in the urine. Once these stones pass into the urinary tract, they can cause significant pain before being expelled from the body. This process often leads to a practical question: will the stone sink or float after it is passed?
The Physical Answer: Do Stones Sink or Float?
Kidney stones almost always sink to the bottom of the toilet bowl due to their composition and high density. They are formed from concentrated mineral salts, which are much heavier than water. The scientific basis for this sinking behavior is that the stone’s density is significantly greater than the density of the surrounding liquid.
The density of water is the benchmark for floating and sinking. Kidney stones are typically composed of materials like calcium oxalate, uric acid, and struvite. Even the least dense stones, such as uric acid stones, have a density that ensures they will not float.
Medical imaging uses Hounsfield Units (HU) to measure density, where water is assigned a value of 0 HU. Calcium-based stones, the most common type, frequently show HU values in the range of 400 to over 600, demonstrating they are substantially denser than water. Uric acid stones are less dense, falling into the 100 to 200 HU range. Because all common kidney stone types possess a higher specific gravity than urine or water, they will sink rapidly upon being passed.
Practical Steps for Retrieval and Collection
Since the stone will sink, the most practical collection method involves filtering the urine at the point of voiding. Healthcare providers often supply a specialized kidney stone strainer, a small device made of fine mesh or gauze designed to catch even very small particles. This straining process should be performed every time you urinate until the stone is passed and recovered.
To successfully retrieve the specimen, the strainer must be held or placed in the toilet to filter all the urine as it is passed. After voiding, the fine mesh should be carefully checked for particles, which may be as small as a grain of sand. Once the stone is located, it should be carefully removed using a clean instrument, such as tweezers, and placed into a clean container.
The stone must be kept dry and placed in a container with a lid, such as one provided in a collection kit. Do not add any liquid, like water or alcohol, as residual moisture can interfere with subsequent laboratory analysis. The stone should be allowed to air dry at room temperature for at least 24 hours before being sealed and returned to the doctor or lab for testing.
Stone Composition and Analysis
Collecting the stone is a necessary step because laboratory analysis is required to determine its exact chemical composition. Knowing the type of stone is paramount, as it dictates the preventative measures, dietary adjustments, and medications necessary to stop future formation. There are four main types of kidney stones: calcium, uric acid, struvite, and cystine.
Calcium stones are the most common, typically comprised of calcium oxalate or calcium phosphate. Uric acid stones tend to be more common in people with a diet high in animal protein or those who do not drink enough water. Struvite stones, composed of magnesium ammonium phosphate, often form after a chronic urinary tract infection.
Cystine stones are the least common type, caused by a hereditary genetic disorder that results in excessive cystine in the urine. The chemical analysis identifies which components make up the majority of the stone. This information allows the healthcare team to create a targeted treatment plan, which may involve specific dietary restrictions, increased fluid intake, or medications to alter the urine’s chemistry, thus managing the risk of recurrence.