What’s Behind Your Lower-Left Back Pain—Kidney Stone, Sacroiliac Joint Pain, or Muscle Strain?

You wake up with a sharp ache low on the left side of your back. Do you ice and stretch and hope it settles? Call your clinician? Or head to the emergency department because it might be a kidney stone? Left-sided low back pain has several common culprits—kidney stone (renal colic), sacroiliac joint dysfunction, and muscular strain—and each has a different “signature,” different risks, and different next steps.

This plain-English guide walks you through:

• How each condition actually feels and where it travels
• Red-flag symptoms that need urgent care
• Simple self-checks (useful, not definitive)
• What to expect at the clinic (tests that confirm the source)
• Treatments that work—and what not to worry about

All advice here is educational and evidence-based, with references so you can read more. If your pain is severe, new, or worrying, seek medical care.

Go straight to urgent care or the emergency department if you have any of the following:

• Fever with flank or back pain (possible kidney infection)
• Severe, colicky pain that makes you pace or cannot get comfortable (possible kidney stone)
• Blood in the urine, pain with urination, or inability to pass urine
• New weakness, numbness, trouble walking, loss of bowel or bladder control, or saddle anesthesia (emergency spine problems)
• History of cancer, recent significant trauma, or unexplained weight loss with back pain

Fever with flank pain and urinary symptoms points to kidney infection (pyelonephritis) and needs prompt evaluation and antibiotics.^[1]

A kidney stone lodged in the ureter typically causes sudden, severe pain that starts in the flank (side of the back below the ribs) and can travel toward the lower abdomen, groin, or inner thigh. The pain often comes in waves as the ureter squeezes against the stone. Nausea or vomiting, restlessness (unable to find a comfortable position), and blood in the urine are common. Some people also feel a frequent urge to urinate or burning when they try.^[2]

Clinicians often describe renal colic as one of the most intense pains people experience. It usually localizes to one side, matching the obstructed ureter, and may shift as the stone moves.^[3]

Important nuance: fever is not typical of a simple stone; fever plus flank pain suggests infection and is urgent.^[1]

The sacroiliac joint connects the spine to the pelvis. Irritation here causes unilateral pain low in the back or deep in the buttock, sometimes radiating to the groin or thigh. It often worsens with sitting, standing from a chair, rolling in bed, or twisting. On exam, clinicians use a cluster of provocation tests—pressures and positions that stress the joint—to see if the pain is reproducible. A family-practice review notes that three or more positive tests increase the likelihood that the sacroiliac joint is the source; targeted local anesthetic blocks can confirm the diagnosis.^[4]

Do not over-trust any single maneuver: newer analyses remind us that test clusters help but are not decisive on their own.^[5]

A muscular or soft-tissue strain along the paraspinal muscles is the most common cause of short-lived low back pain. It typically flares with movement (bending, twisting, lifting) and eases at rest; there are no urinary symptoms or systemic signs. Evidence-based guidelines emphasize staying active, gentle heat, short-term pain control, and reassurance—the majority improve over days to weeks without imaging.^[4]

• Flank (side below the left ribs): think kidney or ureter—especially if pain tracks forward to the groin and comes in waves.^[1]
• Buttock dimple region near the posterior superior iliac spine: think sacroiliac joint—especially if pain flares with sitting, transitions, or twisting.^[4]
• Muscle band next to the spine: think muscular strain—tender to touch, stiff in the morning, movement-sensitive, but without urinary signs.^[4]

Remember: these are patterns, not proofs. Your story plus a focused exam—and sometimes a few tests—separate them with confidence.

• Sudden onset; wave-like escalation; restlessness (hard to lie still).
• Pain often moves from flank toward groin over hours to days; blood in urine can appear.^[1]

• Point with one finger to a spot just inside the dimple of your backside (the “Fortin area”).
• Pain worsens when you sit a long time, stand up, or roll in bed; some feel relief leaning on the other leg. A clinician may reproduce pain with a set of stress tests; three or more positive tests strengthen the case.^[4]

• Sore with pressing on the muscle next to the spine; better with gentle heat and light movement; no urinary symptoms; no fever. First-line care is conservative for most people.^[4]

If you are unsure—or if red flags appear—get evaluated.

1. Urinalysis to look for blood and signs of infection.
2. Imaging guided by your situation:
   • For most non-pregnant adults with significant flank pain and suspicion of a stone, noncontrast computed tomography of the abdomen and pelvis is the preferred test; it shows stone size and location and rules out other serious causes.^[5]
   • Ultrasound is often used first in pregnancy to avoid radiation and can also show signs of blockage in others, though it is less sensitive for tiny stones.^[4]

A “patient-friendly” summary from radiology societies echoes this: computed tomography best defines stones; ultrasound may be chosen initially in select groups.^[6]

What the results mean: stones five millimeters or smaller often pass on their own; stones five to ten millimeters sometimes need help; larger stones often need procedures. Your clinician will discuss pain control and whether a short course of medical expulsive therapy (for example, an alpha-blocker such as tamsulosin) makes sense for a distal ureteral stone in the five-to-ten-millimeter range—evidence supports selective use.^[5]

Pain control: modern reviews and guidelines agree that nonsteroidal anti-inflammatory drugs are appropriate first-line analgesia for renal colic, often reducing the need for opioids.^[7][8]

Be alert for infection: fever, shaking chills, or feeling very unwell with a suspected stone can signal obstructed infection—a true emergency.^[1]

1. History and exam first. Clinicians use a cluster of provocation tests (for example, thigh thrust, compression, distraction, sacral thrust). If three or more are positive, the sacroiliac joint becomes a prime suspect.^[4]
2. If needed, a diagnostic injection of local anesthetic into the joint can confirm it as the pain generator. Evidence warns that test clusters alone have limited accuracy, so clinicians interpret them in context.^[5]
3. Imaging (plain radiographs or magnetic resonance imaging) may be used to rule out other causes or if inflammatory arthritis is suspected; otherwise, many cases are managed based on exam findings.

• In the absence of red flags, major guidelines recommend no immediate imaging. Focus on function: stay as active as tolerated, consider heat, short courses of nonsteroidal anti-inflammatory drugs or other evidence-based options, and begin gentle mobility and strengthening as symptoms settle. Most improve within a few weeks.^[4]

Imagine three different stories:

• Hydration as advised by your clinician and nonsteroidal anti-inflammatory drugs for pain (unless you have a reason to avoid them).
• Discuss medical expulsive therapy if your imaging shows a distal ureteral stone in the five-to-ten-millimeter range; selective short-term use of an alpha-blocker like tamsulosin can improve passage rates in this group.^[5]
• Follow-up to ensure passage and to prevent future stones. Many stones pass without procedures, but persistent obstruction, uncontrolled pain, infection, or larger stones often need urologic intervention. Computed tomography or ultrasound helps guide decisions.^[5]

• Activity modification (limit aggravating twists or long sitting periods).
• Physical therapy focused on pelvic stability: gluteal and core strengthening, hip mobility, and movement re-education; manipulative therapy can be helpful in select patients.^[4]
• Short-term pain control (for example, nonsteroidal anti-inflammatory drugs if appropriate).
• If pain persists and the exam remains sacroiliac-focused, your clinician may discuss image-guided injection for diagnosis and relief. Evidence supports using injections after a careful exam and conservative trial.^[4]

• Keep moving (avoid bed rest), sprinkle in gentle heat, and use short-term nonsteroidal anti-inflammatory drugs or acetaminophen if safe for you.
• After a few easy days, start graded activity: walking, hip hinges with light loads, and simple core work.
• Most episodes recover within days to weeks; imaging is not recommended early unless red flags appear.^[4]

Not every left-sided ache is kidney, sacroiliac, or muscle. A small subset will be nerve-related (lumbar disc or foraminal stenosis), hip pathology, shingles in its early phase, or visceral sources (for example, kidney infection rather than stone). Fever, systemic symptoms, pain that wakes you at night without movement triggers, or progressive neurologic changes should prompt re-evaluation. Clinicians rely on red-flag screens to decide who needs earlier imaging.^[4]

Yes. While hematuria, urgency, or burning are common, some people present with flank-predominant pain alone. The classic colicky pattern and radiation toward the groin help distinguish it. A urinalysis and, when appropriate, computed tomography or ultrasound sort it out quickly.^[1]

Kidney infection usually adds fever, chills, malaise, and sometimes costovertebral angle tenderness on exam, along with urinary symptoms. Stones typically do not cause fever unless there is also an infection—an emergency.^[1]

Used as a cluster, they increase confidence, and a diagnostic injection can confirm. But studies show clusters alone are not definitive, which is why clinicians interpret them alongside your history and exam.^[4]

Usually no. Multiple guidelines recommend no early imaging for uncomplicated mechanical low back pain; focus on staying active and symptom relief. Imaging comes later if red flags are present or pain persists beyond several weeks.^[4]

1. Is the pain colicky and wave-like, moving from flank toward the groin?

   • Yes: think kidney stone → seek evaluation the same day (especially if severe). Urinalysis plus computed tomography or ultrasound confirms and guides treatment.^[5]

2. Is the pain deep in one buttock, worse with sitting, standing up, or rolling in bed?

   • Yes: think sacroiliac joint → see a clinician or physical therapist familiar with provocation clusters and pelvic stabilization work.^[4]

3. Is the pain movement-sensitive and localized to the paraspinal muscles, without urinary signs or fever?

   • Yes: likely muscular strain → keep moving, use heat and short-term analgesia if safe; reassess if not improving. ^[4]

4. Any red flags (fever, neurologic deficits, inability to pass urine, cancer history, trauma)?

   • Yes: urgent evaluation. ^[4]

Kidney stone pain is sudden, severe, one-sided, wave-like, and often radiates toward the groin, with possible blood in the urine. Urgent evaluation rules out infection and guides pain control and passage. Computed tomography is the go-to imaging for most non-pregnant adults; ultrasound is preferred initially in pregnancy. Nonsteroidal anti-inflammatory drugs are first-line for pain.^[5][4]

Sacroiliac joint pain sits low and lateral, hates sitting and twisting, and may radiate to the groin or thigh. Provocation test clusters and (when needed) a diagnostic injection help confirm; pelvic-stability therapy typically helps.^[4]

Muscular strain is movement-sensitive and self-limited. Most people recover with activity, heat, and short-term analgesia—and no early imaging.^[4]

When you map your exact pain pattern and pair it with the right next step, you shorten the road back to normal life—and avoid unnecessary worry.

References:

Educational content only; not a substitute for individualized medical care. If you have severe or uncertain symptoms, seek medical help.