Anyone who has had a kidney stone remembers the pain. It arrives without warning, comes in waves, and gets described as the worst thing the person has ever felt.
What is less well known is that the scan is not looking for the pain. It’s looking for a number, and that number decides whether you go home to wait or need something done.
We are a freestanding emergency room in Pantego with CT on site, open every hour of every day. Arlington ER sees stone pain most weeks.
The Short Answer: Size Decides What Happens Next
A CT scan shows how large a stone is, where exactly it sits, and whether it is backing urine up into the kidney.
Size is the number that drives everything else. Small stones usually pass on their own. Larger ones often don’t. That’s why CT is the test rather than one option among several.
Why Is CT the Test for Kidney Stones?
Because it finds almost all of them, and it measures them well.
A specific low-dose protocol is used, without contrast dye. Stones show up because they are dense, and the scan gives their size in millimeters plus their exact position along the tube running from kidney to bladder. It also shows whether that tube is blocked and whether pressure is building behind it, which is the finding that turns a painful night into an urgent problem.
Ultrasound can find stones too, and it is preferred in pregnancy and often in children because it uses no radiation. Its limitation is precision: it can struggle to size a stone, or to see one sitting partway down toward the bladder, and size is the whole point.
Ordinary X-ray isn’t reliable here, since some stones don’t show up on it at all.
What Does the Scan Actually Show?
Four things, and each changes the plan.
- Size, in millimeters, which predicts whether the stone passes without help
- Position, since a stone near the bladder has nearly finished its journey while one still in the kidney has not started
- Obstruction, meaning whether urine is backing up and the kidney is swelling under pressure
- Anything else, because a scan looking for stones sometimes finds a different explanation for the same pain
That last point matters more than it sounds. Stone pain can resemble several other conditions, some of them surgical, and a scan that rules those out is doing real work even when it finds the stone everyone expected. That is the same reasoning behind how we approach abdominal pain emergency care.
Does Size Really Decide Everything?
It decides most of it. Broadly, the smaller the stone, the better the odds it passes on its own.
| Stone size | Likely to pass on its own? | What that usually means |
| Under 4 mm | Most do | Fluids, pain control, and time at home |
| 4 to 6 mm | Many do, more slowly | Home management, with urology follow-up arranged |
| Over 6 mm | Often not | Urology involvement, a procedure is frequently needed |
| Any size, with obstruction | Not the deciding factor | Treated urgently regardless of size |
Position modifies all of this, and so does whether the kidney is obstructed. A blocked kidney with infection behind it is an emergency at any stone size, and that is the situation the scan exists to catch.
Which Symptoms Mean Go Now Rather Than Wait?
Stone pain alone is miserable but not always urgent. These change that:
- Fever or chills alongside the pain, which can mean infection behind a blockage
- Vomiting that stops you keeping fluids down
- Pain that no position and no amount of waiting eases
- Passing very little urine, or none
- Only one working kidney, or a transplanted kidney, with any stone symptoms at all
The fever combination is the one to take most seriously. An obstructed, infected kidney can deteriorate fast and needs the blockage relieved rather than time. A fever with flank pain is a same-day emergency visit, not a wait-and-see.
What Else Happens During the Visit?
Bloodwork and a urine test run alongside the scan, because the picture and the numbers answer different halves of the question.
Urine typically shows blood, often microscopic, which supports the diagnosis, and it shows infection if one is present. Blood tests show how the kidneys are working, which matters if one is obstructed. All of it is processed here through on-site lab testing, so results and images arrive together rather than hours apart.
Pain relief isn’t an afterthought either. It’s a priority and it starts before the scan, because there is no clinical reason to make someone wait in that state for an image.
Where imaging is the right next step, what a scan involves is detailed on our page covering emergency CT scan services, and the full picture of how stones are assessed and treated sits on our kidney stone emergency care page.
What Happens After You Go Home?
Most people with a small stone are managed at home, with fluids, pain control, and instructions on what to watch for. Straining urine to catch the stone is sometimes suggested, because what it is made of can guide prevention.
Come back if a fever develops, if vomiting stops you keeping fluids down, if the pain escalates beyond what you were sent home to manage, or if you stop passing urine.
Prevention is worth taking seriously, because kidney stones recur in a substantial share of people within a few years. Fluid intake is the single most effective lever. Dietary changes depend on what the stone was made of, which is a conversation for urology follow-up rather than a general rule.
Frequently Asked Questions
It’s unusual. The protocol used for stones detects the overwhelming majority, including very small ones. The more common scenario is the scan finding no stone because the pain has a different cause, which is useful information rather than a failed test.
Anywhere from a few days to several weeks, depending mostly on size and position. Smaller stones sitting closer to the bladder move fastest. Pain that comes in waves during that period is expected. Pain with fever is not.
The protocol used for stones is a reduced-dose one, specifically because people with stones often need imaging more than once in their lives. The radiation dose is not zero, which is why it is not ordered without a reason, and it is why ultrasound is chosen instead during pregnancy.
Staying well hydrated helps a small stone move along and helps prevent new ones forming. It will not dissolve a stone or push out one too large to pass.
An untreated blockage can, especially when infection is involved, which is why obstruction is treated urgently regardless of stone size. A stone that passes on its own normally leaves no lasting harm.
