After ignoring painful urination for a few days, you suddenly develop a high fever, full-body chills, and pain on one side of your lower back so severe you can barely touch it. When a urologist sees this combination, the first thing that comes to mind is acute pyelonephritis. Unlike simple cystitis, this is not something that will go away just by drinking more water and toughing it out—you usually need medical attention right away.
What is acute pyelonephritis?
Acute pyelonephritis occurs when bacteria reach the kidney, causing inflammation of both the kidney tissue itself and the renal pelvis, which collects urine. It is the most common upper urinary tract infection. Most cases affect only one side, though occasionally both sides are involved.
You can think of it this way: in cystitis, the infection stays in the bladder, whereas in pyelonephritis, bacteria travel all the way up into the kidney. The higher up the infection goes, the stronger your body’s reaction usually is.
Common symptoms: fever, chills, and one-sided flank pain
There are three classic signs:
- Fever and chills, often with noticeable shivering
- Pain on one or both sides of the lower back, with costovertebral angle tenderness when tapped gently below the back ribs
- Urine tests showing pyuria and bacteria
Many people experience symptoms of cystitis beforehand or at the same time, such as frequent urination, urgent urination, painful urination, blood in the urine, or cloudy, foul-smelling urine. Others mainly have stomach pain, nausea, and vomiting, which can initially be mistaken for a stomach flu.
Keep in mind that older adults, people with diabetes, or those with weakened immune systems may not develop much of a fever. Instead, they might only seem tired or lose their appetite, making the infection easy to overlook.
How do bacteria reach the kidneys?
The most common route is an upward climb: bacteria enter the bladder through the urethral opening and then travel up the ureter to infect the kidney. Spread through the bloodstream or lymphatic system is much less common.
The bacteria responsible are mostly intestinal bacteria, most commonly E. coli. Certain strains are especially good at sticking to the lining of the urinary tract, and some people are naturally more prone to having these bacteria attach, which is why some individuals experience repeated infections.
Women are affected more often than men, mainly because the female urethra is shorter and its opening is closer to the anus; sexual activity can also introduce bacteria into the urethra.
Who is at higher risk of severe infection?
The following conditions make the infection more likely to happen and harder to clear:
- Urinary tract obstruction or structural abnormalities, such as stones, hydronephrosis, or benign prostatic hyperplasia
- Vesicoureteral reflux (where urine flows backward toward the kidneys, seen more commonly in children)
- Diabetes, especially with poorly controlled blood sugar
- A weakened immune system, such as undergoing chemotherapy or long-term immunosuppressive therapy
- Pregnancy
- Long-term use of a urinary catheter
In people with these conditions, even less aggressive bacteria can trigger an infection.
What tests are needed?
- Urinalysis: Checks for white blood cells, red blood cells, and bacteria in the urine.
- Urine culture: Identifies the exact type of bacteria and which antibiotics will work against it. This is very important because antibiotic resistance is becoming increasingly common.
- Blood tests: Check your white blood cell count, inflammation markers, and kidney function. If you have a high fever, a blood culture will also be ordered.
- Kidney ultrasound: The most commonly used imaging test, mainly to check for blockages, kidney stones, or hydronephrosis.
- CT scan: Not everyone needs one. It is usually arranged only if symptoms are severe, a fever does not come down after treatment, or a renal abscess or blockage is suspected.
- Voiding cystourethrography: Used to check for vesicoureteral reflux. It is usually done after the infection is under control and is mainly considered in children with recurrent infections.
Treatment: Why Is Hospitalization Often Needed?
The core of treatment is antibiotics, along with plenty of fluids and medication to reduce fever and relieve pain.
If your symptoms are mild, you can keep food and fluids down, and you have no other high-risk factors, your doctor may let you recover at home with oral medication, along with scheduled follow-up visits. However, if you have a high fever, persistent vomiting, or dehydration, or if you have diabetes, are pregnant, or have a urinary tract blockage, hospitalization for intravenous fluids and treatment is usually recommended until the fever subsides and your condition stabilizes before switching to oral medication.
You must finish the full course of medication exactly as prescribed by your doctor. Stopping your medication on your own as soon as the fever goes down is a very common cause of relapse. If tests reveal a kidney stone or blockage, medication alone is often not enough—the blockage must be relieved first, such as by placing a drainage tube or treating the stone, before the infection can be brought under control.
With prompt and appropriate treatment, most people recover well without lasting kidney function problems.
Risks of Delayed Treatment: Emphysematous Pyelonephritis and Sepsis
If left untreated, incompletely treated, or if an underlying blockage is not addressed, the infection can worsen: pus can collect in the kidney to form a renal abscess, and people with diabetes may develop emphysematous pyelonephritis (gas forming inside the kidney). If bacteria spread into the bloodstream, it causes sepsis, which in severe cases can lead to septic shock—a life-threatening condition.
This is why you should not wait and watch at home if you have both a fever and flank or lower back pain.
When Should You Seek Immediate Medical Attention?
- Fever and chills accompanied by flank or lower back pain or urinary symptoms
- Vomiting so severely that you cannot keep food or water down
- Confusion or decreased alertness, shortness of breath, low blood pressure, or cold hands and feet
- Experiencing any of the above symptoms while pregnant
- Persistent fever that does not come down despite treatment
- Having diabetes, kidney stones, a single kidney, or a weakened immune system
How Can You Prevent Recurrence?
The most important step is to find and address the underlying cause. If you have kidney stones, a blockage, or reflux, treating those issues is essential to prevent the infection from coming back. Everyday habits that also help include:
- Drink plenty of water and do not hold in your urine
- For women, wipe from front to back after using the toilet
- Urinate after sexual intercourse
- Keep blood sugar well controlled if you have diabetes
- Seek medical care early when symptoms of cystitis appear, and do not delay
For people with recurrent infections, a doctor may discuss preventive medication options; whether and how to use them requires evaluation based on your individual condition.
Advice from a Urologist
Acute pyelonephritis sounds frightening, but with early treatment, most cases can be cured. What we worry about most are two situations: first, mistaking it for a cold or gastroenteritis and delaying care; second, stopping medication on your own as soon as the fever goes down. Also, if it keeps happening to the same person, there is usually an underlying cause that is worth a thorough evaluation.
Acute Pyelonephritis FAQ
Both are urinary tract infections, and the difference lies in the location. Cystitis usually causes only frequent urination and painful urination, rarely with a high fever; pyelonephritis means the infection has reached the kidney, commonly causing fever, chills, and flank or back pain, which is more serious and requires prompt medical attention.
Not necessarily. People with mild symptoms who can eat and drink normally and have no other risk factors may be treated at home with oral medication. Hospitalization is usually recommended for those with persistent high fever, vomiting and dehydration, pregnancy, diabetes, or urinary tract obstruction.
It is not recommended. A drop in fever only means the infection is coming under control, not that the bacteria have been completely cleared. Stopping medication on your own makes recurrence likely and may lead to antibiotic resistance, so please complete the full course as directed by your doctor.
Not necessarily. More common causes of lower back or flank pain are musculoskeletal issues or kidney stones. However, if back or flank pain is accompanied by fever, chills, or urinary symptoms, pyelonephritis should be considered, and you should seek medical attention promptly.
With early and appropriate treatment, most people do not have lasting kidney function problems. Greater concern arises when there are recurrent infections, delayed treatment, or untreated obstruction or reflux.
Common causes include urinary stones, urinary tract obstruction, vesicoureteral reflux, poorly controlled blood sugar, or a habit of holding in urine. People with recurrent infections are advised to visit a urologist for a comprehensive evaluation to identify the underlying cause.
Further Reading
- What Causes Cystitis?
- What Causes Back Pain? Is Flank Pain a Kidney Stone?
- What Are the Symptoms of Urinary Tract Stones?
- What Causes Hydronephrosis?
- What Causes Painful Urination?
Medical References
This article provides general health education and cannot replace a doctor’s diagnosis. Please seek medical evaluation if you feel unwell.
