Kidney infection

Also called Pyelonephritis, Upper urinary tract infection

A urine infection that has reached the kidney. Loin pain, fever and rigors turn a common problem into an urgent one.

What it is

Infection of one or both kidneys, usually from bacteria travelling up from the bladder.

It is more serious than a simple bladder urinary tract infection: it makes people systemically unwell, it can cause sepsis and acute kidney injury, and it needs a longer course of antibiotics, typically seven to 14 days rather than three.

The distinguishing features are systemic: fever, shivering attacks called rigors, back or side pain, and feeling genuinely ill, rather than just stinging and frequency.

Certain groups need lower thresholds and more urgent assessment: anybody pregnant, where it risks preterm labour; men, in whom it is less common and more likely to indicate an underlying problem; children, in whom it can cause kidney scarring; and anybody with diabetes, immunosuppression, kidney stones or a catheter.

Recurrent kidney infections, or infection in a man or a child, warrant imaging to look for an obstruction, kidney stones or reflux.

A blocked, infected kidney, usually from a stone, is a urological emergency needing drainage, not just antibiotics.

Signs you might notice

Fever, chills and shivering attacks.

Pain in the back or side, below the ribs, usually on one side.

Feeling generally very unwell, nauseated and often vomiting.

Bladder symptoms: burning, frequency, urgency, cloudy or bloody urine, though these may be absent.

Confusion, particularly in older people, sometimes as the only sign. See delirium.

999 or A and E: confusion, very fast breathing, mottled skin, not passing urine, a rash that does not fade, or feeling like something is very wrong. See sepsis.

Same-day: fever with loin pain; any suspected infection in pregnancy; inability to keep fluids or antibiotics down; a man or a child with these symptoms.

How it can affect day-to-day life

In an older person a kidney infection often presents as confusion and falling rather than as anything urinary, and the classic symptoms may be entirely absent.

Equally, cloudy or smelly urine alone in an older person without other symptoms is not a reason for antibiotics; asymptomatic bacteria in the urine are common and treating them causes harm. The distinction is whether the person is systemically unwell.

Dehydration during the illness compounds kidney damage, so fluids matter as much as antibiotics.

Catheters are a common route in, and long-term catheters are almost always colonised, so the decision to treat should be based on illness, not on a dipstick.

After treatment, drinking well, not delaying passing urine, and treating constipation all reduce recurrence.

Supporting someone well

Get a same-day assessment for fever with back or side pain.

Provide a urine sample before antibiotics start, so treatment can be adjusted.

Complete the full course, which is longer than for a simple bladder infection.

Keep fluids going; dehydration worsens the kidney injury.

Learn the sepsis signs and act on them.

In an older person, treat new confusion as a possible infection and get them seen; do not treat a dipstick alone.

For a man or a child, expect further investigation rather than repeated courses.

For recurrent infections, ask about imaging and about prevention rather than antibiotics on repeat.

Review medicines that need holding during acute illness. See acute kidney injury.

Where to get help

A GP practice or NHS 111 the same day.

999 or A and E for suspected sepsis, or inability to keep fluids down.

Urology urgently for a suspected obstructed, infected kidney.

Kidney Care UK run a helpline and cover urinary infections and kidney health.