What Are Kidney Stones?

Overview

Urine is a liquid waste product produced by the filtration of blood by the kidneys. Urine is made of surplus water containing many different dissolved chemicals, including urea, salts and creatinine. This fluid passes from the kidney through the urinary system and is excreted from the body via urination. This process helps to maintain the optimal balance of chemicals and water in the blood and keeps it clean. 

Under certain conditions, the dissolved chemicals present in urine can crystallise. Small crystals can join together and attract other organic matter, resulting in the formation of larger solid deposits, known as kidney stones. 

So, kidney stones are solid masses created by this crystallisation and clumping together of chemicals in the urine. Formed in the kidney, these stones may move into the rest of the urinary system through the ureters (tubes connecting kidneys and bladder), bladder and urethra (tube leading from the bladder through which urine leaves the body). 

Kidney stones are usually irregular in shape and can have either smooth or jagged surfaces. According to the US National Institute of Diabetes and Digestive and Kidney Disease (NIDDK), kidney stones can range from the size of a grain of sand to the size of a pea. In rare cases, they can reach the size of a golf ball. You can have just one kidney stone or several smaller ones.

The smaller stones may pass through the urinary system and be excreted from the body in the urine without problems or symptoms. However, larger ones can cause severe pain and urinary blockage and may result in infection and damage to the urinary system.

As the most common disorder of the urinary system,¹ Kidney stone will be experienced by 8.8% of people (in the US) during their lifetime.² You are at greater risk of having a kidney stone if you have had one previously or have a family history of them.

Kidney stones are known by several other names, including renal stones, renal calculi,³ nephroliths, nephrolithiasis and urolithiasis.⁴

Types of kidney stone

Kidney stones can be formed from different chemicals. The most common ones are:¹

1. Calcium stones – make up 80% of all stones.⁴ Formed of calcium oxalate (most commonly) or calcium phosphate, or a combination of both chemicals 
2. Struvite stones – are usually caused by a urinary tract infection when bacteria produce ammonia, which combines with other chemicals in the urine to form solid magnesium ammonium phosphate.
3. Uric acid stones – are formed in acidic urine. The acidity can be due to diet or to other underlying health conditions. A predisposition to uric acid stones can ‘run in families’
4. Cystine stones – are rarely found and are caused by the inherited genetic defect, cystinuria, which increases levels of the amino acid cysteine in the urine. Again, this can run in families.
5. Other – in rare cases, stones can be formed from other chemicals

Causes of kidney stones

Kidney stones are formed by the crystallisation of chemicals in urine. According to The British Association of Urological Surgeons, this process is a consequence of:

• High concentration of certain chemicals in urine
• Insufficient levels of chemicals that prevent crystallisation in urine 
• Insufficient volume of urine 

The formation process is complex, often multifactorial, and can differ from person to person. Contributory factors include diet, lifestyle, genetics, medications, and other health conditions.

Risk factors for kidney stones

The composition and concentration of urine, and so the likelihood of crystal formation, is influenced by many different processes in the body, so risk factors are many and include:

Genetic risk factors: 

• People who are assigned male at birth (AMAB) are more likely to have stones than those assigned female at birth (AFAB)
• Personal history of previous stones
• Family history of kidney stones³

Dietary risk factors:

• Insufficient fluid intake 
• Low calcium intake
• Low fruit and vegetable intake
• High sodium intake
• High animal protein (meat and fish) intake
• High oxalate intake.⁵ (further information on high oxalate foods here) 

Other health conditions:

There are a wide range of health conditions which can increase the risk of kidney stone formation. These include obesity, diabetes, hypertension (high blood pressure) and metabolic syndrome.⁶ Conditions that affect chemical levels in blood or urine and others that influence the absorption of fluid and nutrients by the digestive system may also increase risk. Conditions affecting the kidney and urinary system – including structural abnormalities, kidney disease and recurrent urinary tract infections – may also be contributory. 

More information can be found here: The US National Institute of Diabetes and Digestive and Kidney Diseases

Medications:

There are a small number of medications that may increase the risk of developing kidney stones, including common medicines such as aspirin, antacids, and diuretics. A list of these can be found on the NHS website.

Signs and symptoms of kidney stones

Stones that stay in the kidney may not produce any symptoms. However, once a stone leaves the kidney and enters the rest of the urinary system, it must pass through narrow tubes, and this can cause urinary obstruction and hydronephrosis (swelling of the kidney due to build-up of urine), spasms of the ureter or bladder muscles, and infections of the kidney or urinary tract. 

The following symptoms may result: 

• Pain - acute, often severe pain in the flank (the side of the lower back), possibly radiating to the abdomen, groin, testicle and labia. Pain may fluctuate in intensity, coming in waves,⁷ and may change as the stone passes through different parts of the urinary system. Intense pain experienced in waves is called renal colic

• Nausea and vomiting³
• Changes in urination include:
  • Pain/burning sensation
  • Presence of blood in urine
  • Urine which is cloudy or has an unusual smell
  • Intense need to urinate and frequent urination
  • Difficulty urinating

• Fever and chills⁷
• Costovertebral tenderness³

Diagnosis of kidney stones

The following procedures may be used to confirm a diagnosis of kidney stones and exclude other conditions that can present with similar symptoms:

• History taking, including:
  • Current general health and past medical history
  • Family history of kidney stones
  • Diet and lifestyle, including intake of fluid, protein, sodium, high oxalate foods, fruit and vegetables, and supplements 

• Physical examination, including examination of the abdomen and back for signs such as:
  • costovertebral angle tenderness (the space between the bottom rib and the spinal column in the back)⁸
  • hypoactive bowel sounds (reduction in bowel sounds indicative of an underactive digestive system)⁷

• Urine tests – carried out with a urine dipstick or by laboratory analysis to check urine for:
  • Acidity (pH) level
  • Presence of blood
  • Evidence of infection
  • Presence of crystals
  • Levels of the chemicals that are involved in stone formation

Certain tests can be carried out using an isolated single sample. However, a more detailed analysis may require urine collection over a 24-hour period.

• Blood tests to check:
  • Kidney function
  • For other underlying health issues
  • Levels of chemicals that are involved in stone formation 

• Imaging tests:
  • X-ray KUB – 2-dimensional imaging of the kidney, ureter and bladder using X-ray 
  • CT KUB – three-dimensional imaging of the kidney, ureter and bladder using CT scan (Computerised Tomography). 
  • Intravenous pyelogram – X-ray images taken after injection of contrast dye 
  • Ultrasound – although this technique has diagnostic limitations with kidney stones, it can be useful in cases when other imaging techniques may be contraindicated, for example, during pregnancy. 

CT KUB is considered the most accurate way of confirming diagnosis.⁷ It can also provide detailed information about the stone – its size, shape, how hard or soft it is likely to be, and where it is located within the urinary tract – which is all useful in guiding treatment choices. 

• Analysis of stone once passed – to determine the chemicals involved in its formation, which may indicate certain underlying processes or health conditions. 

Management and treatment for kidney stones

Stones will often pass on their own, exiting the body in the urine, particularly those that are small in size (less than 5mm).⁷ This process may be aided by:

• Additional fluids – possibly supplied by IV (intravenously)⁷
• Pain relief – often in the form of NSAIDS (non-steroidal anti-inflammatories)^{3, 9}
• Alpha-blocker medications – used to relax the ureter muscles – mostly effective for larger stones^9,10
• Antibiotics – to treat infections when present.⁷
• Antiemetics⁷ – to reduce nausea and vomiting
• 80-90% of stones will pass in this way without more significant intervention.⁷ Follow-up imaging may be used to track progress.⁹ 

Larger stones, or those causing infection or obstruction, may require more intervention. The treatment chosen will depend on the size, shape and location of the stone, as well as general health issues and current medication used in each case. 

There are a number of procedures that can be used either to remove the stone or to break it up in order to allow it to pass out of the body into urine. These include:

• Extracorporeal shock wave lithotripsy – a non-invasive procedure involving the use of sound waves to create vibrations which break a stone into smaller pieces, allowing it to pass out of the body in urine
• Percutaneous nephrolithotomy and percutaneous nephrolithotripsy – surgery to remove kidney stones involving a small incision made in the back. The stone is removed either whole (Nephrolithotomy) or after being broken into smaller pieces (Nephrolithotripsy). It is usually performed under general anaesthesia.
• Ureteroscopy – insertion of a small scope (a thin, flexible tube with a camera at the end) into the urinary system via the urethra to find and remove a kidney stone, either whole or after breaking it up into smaller fragments.

Prevention of kidney stone recurrence

Once an acute episode is resolved, patients may be treated to prevent recurrence of kidney stones. The choice of preventative treatment will depend on the type of stone and what has caused it. This is determined by an analysis of blood, urine and stone composition, and may include:

• Diet and lifestyle changes to optimise chemical levels in the blood, maintain optimal body weight, balance blood sugars and reduce hypertension, such as:
• Drinking a sufficient volume of fluids daily reduces the concentration of chemicals in urine and prevents crystal formation. 2.5 - 3 litre per day is recommended⁹ or sufficient to produce 2 litres of urine per day^{1, 3, 7}
• Maintaining calcium intake, neither significantly above or below recommended levels* 
• Reducing sodium intake* 
• Reducing intake of animal products 
• Increasing intake of fibre and vegetables
• Reducing carbonated drink consumption 
• Reducing intake of refined sugar
• Drug therapy to address metabolic imbalances and other underlying conditions, for example:
  • Citrate supplements⁹ to reduce the acidity of urine
  • Thiazide diuretics⁷ to help the body to get rid of water
  • Allopurinol⁷ to reduce levels of uric acid in the blood. 

*Calcium stones are the most common type of kidney stone, and high concentrations of calcium in urine are a risk factor for their formation. However, this is not necessarily caused by a high intake of calcium in food and drink. Instead, high calcium levels in urine may be due to high sodium intake. When the kidneys filter out excess sodium from blood, calcium is pulled across with it into the urine. Reducing sodium intake may, therefore, be a more effective way of reducing calcium levels in the blood than reducing calcium intake. Calcium is important for the body in many ways, and intake should be kept at recommended levels.

The specific recommendations for dietary changes will depend on the individual case, and a Dietician may be able to provide these. More information can be found on the National Institute of Diabetes and Digestive and Kidney Diseases website. 

Complications of kidney stones

The National Kidney Foundation states that kidney stones can be a cause of chronic kidney disease and (rarely) sepsis, a life-threatening condition resulting from an infection. 

FAQs

How can I prevent kidney stones?

Kidney stone formation is complex and multifactorial, with a number of potential contributory factors. However, general preventative recommendations include:

• Ensuring adequate fluid intake of 2.5-3 litres per day
• Reducing intake of salt, sugar, animal protein and oxalate-rich foods
• Maintaining optimal calcium intake, ideally through food rather than supplements
• Maintaining an optimal body weight

How common are kidney stones?

Kidney stones are a common urological condition, affecting 8.8% of the US population in their lifetime, and more common in people with AMAB than those with AFAB (10.6% compared to 7.1%).² 

The number of people experiencing kidney stones seems to be increasing overall.^{2, 9} You are more likely to have a kidney stone if you have had one previously or have a family history of it. 

What can I expect if I have kidney stones?

Kidney stones that stay in the kidney, or small stones which pass through the urinary system easily, may not cause any symptoms. However, larger stones can cause severe pain as they pass through the system and may result in urinary obstruction and infection. 

When should I see a doctor?

Kidney stones can cause pain, commonly in the flank (the side of the lower back), possibly radiating to the abdomen or groin. Pain may be severe and come and go in waves. Nausea and vomiting may also occur, as well as fever and chills. If you experience these symptoms, consult your doctor. 

Summary

Kidney stones are solid masses which form in the urine through the crystallisation of chemicals. They can range in size, and small ones may pass out of the body unnoticed. However, larger ones can cause severe pain, urinary tract obstruction and infection. There are a number of underlying things which contribute to their formation, including diet and lifestyle factors.