Most kidney stones pass on their own and, while extremely painful, don’t cause lasting harm. But kidney stones can become genuinely dangerous when they block urine flow for an extended period, trigger an infection, or go undetected long enough to damage the kidney. The mortality rate for a kidney stone complicated by septic shock ranges from about 9% to 27%, making infected, obstructing stones a medical emergency.
For the majority of people, a kidney stone means a few days of intense pain followed by relief. Understanding where the line falls between “painful but manageable” and “potentially life-threatening” can help you recognize when a stone needs urgent attention.
Why Kidney Stones Cause Damage
A kidney stone becomes dangerous not because of the stone itself, but because of what it does to urine flow. When a stone lodges in the ureter (the narrow tube connecting your kidney to your bladder), urine backs up into the kidney. This causes the kidney’s internal structures to swell, a condition called hydronephrosis. The rising pressure disrupts the kidney’s ability to filter blood, alters blood flow to the organ, and can even force urine to leak backward into the kidney’s veins and lymphatic system.
If the blockage clears within roughly two weeks, the kidney typically recovers full function. Longer obstructions start to cause permanent problems. The tubules inside the kidney, which do the actual work of filtering waste, begin to shrink and die. Once enough of these structures are lost, the damage is irreversible even after the stone is removed. This is why a stone that causes no symptoms can sometimes be more dangerous than one that sends you to the emergency room in agony: pain is what prompts treatment.
The Real Risk: Infection Plus Obstruction
The most dangerous scenario with kidney stones is when a blocked ureter traps bacteria behind the stone. Urine that can’t drain becomes a breeding ground for infection. Bacteria and urease-producing organisms break down urea into ammonia, creating an alkaline environment that promotes further stone growth while fueling a worsening infection. If that infection enters the bloodstream, it becomes urosepsis.
Hospital mortality for patients with septic shock from an obstructing ureteral stone reaches 27.3%, compared to about 11% for urinary sepsis without an obstruction. The combination of blocked drainage and active infection is what makes this so lethal: antibiotics alone can’t clear an infection when infected urine has nowhere to go. Emergency drainage of the kidney, either through a stent or a tube placed through the back, becomes necessary before the infection can be controlled.
Warning signs that a kidney stone has become infected include fever, chills, cloudy or foul-smelling urine, and feeling generally ill beyond what the pain alone would explain. These symptoms alongside flank pain warrant an immediate trip to the emergency department.
Stones That Don’t Hurt Can Still Cause Problems
Not all kidney stones announce themselves with the classic excruciating pain. Some sit in the kidney or ureter causing a partial blockage without obvious symptoms. Research tracking patients with these “silent” stones found that within four years, 55% experienced stone growth, 48% developed pain, and 26% ultimately needed a procedure to remove the stone. Altogether, 77% experienced at least one of these outcomes. A stone that isn’t bothering you today has a high likelihood of becoming a problem eventually, and in the meantime, a partial obstruction can quietly erode kidney function.
Who Faces Higher Risk
Kidney stones during pregnancy carry unique dangers. Symptomatic stones occur in roughly 1 in every 250 to 1,500 pregnancies, most often in the second and third trimesters. Beyond pain, stones during pregnancy are linked to preeclampsia, urinary tract infections, preterm labor, and in rare cases, pregnancy loss. Treatment options are more limited during pregnancy because radiation from CT scans and certain procedures pose risks to the fetus, making diagnosis and management more complicated.
People with a single functioning kidney face amplified danger from any obstruction, since there’s no second kidney to compensate. The same applies to anyone with pre-existing kidney disease, a kidney transplant, or bilateral stones blocking both sides simultaneously. In these situations, even a brief obstruction can push kidney function into a critical range.
How Common Kidney Stones Are
Kidney stones are not rare. In 2021, there were an estimated 106 million new cases of kidney stones globally, a 26.7% increase from the year 2000. Rising rates are particularly notable in Central America, the Caribbean, and tropical regions of Latin America. The increase is driven partly by diet, rising obesity rates, and climate change pushing more people into chronic mild dehydration.
When Stones Need Medical Procedures
Most stones under 5 millimeters pass without intervention. Larger stones, or those that won’t budge after a reasonable period, typically require one of two main approaches: shockwave lithotripsy, which uses focused sound waves from outside the body to break stones into passable fragments, or ureteroscopy, where a thin scope is threaded up through the urinary tract to extract or laser the stone directly.
Both procedures carry a similar overall complication rate of about 22.5%. Most complications are minor: temporary blood in the urine, soreness, or short-lived urinary symptoms. Serious complications like injury to the ureter or post-procedure infection are uncommon but possible, which is why procedures are generally reserved for stones that pose more risk by staying put than by being removed.
Red Flags That Need Emergency Care
A kidney stone crosses from painful nuisance to emergency when any of the following are present:
- Fever or chills alongside flank pain, suggesting the stone has caused or is worsening an infection
- Inability to keep fluids down due to nausea and vomiting, which leads to dehydration and makes passing the stone harder
- Decreased urine output or no urine at all, which may indicate bilateral obstruction or a blockage in someone with a single kidney
- Confusion, rapid heart rate, or feeling faint, which can signal sepsis developing from an infected stone
Pain alone, even severe pain, is not typically dangerous. But pain combined with fever is a different situation entirely. The infection behind an obstructing stone can progress from manageable to life-threatening within hours, and early intervention dramatically improves outcomes.