Kidney infection:
warning signs, antibiotics & when it gets urgent

1. What is a kidney infection?

In a kidney infection (pyelonephritis) bacteria have reached not only the bladder but the renal pelvis and the kidney tissue itself. They usually travel up through the urethra, the bladder and the ureter; the most common organism is Escherichia coli from your own bowel.^¹

The difference from a bladder infection: there a lining is irritated, here a whole organ is inflamed — with fever, shivering chills and a real sense of being ill. Tea and waiting it out will not deal with this. Alongside the acute form (N10) there are chronic courses (N11); more important, though, is this division:

• Uncomplicated: healthy, non-pregnant women with nothing unusual about the urinary tract.
• Complicated: everything else — stones, catheters, obstructions to drainage, weakened defences, pregnancy and every man: in men any urinary tract infection counts as complicated, because there is almost always an anatomical or prostate-related cause behind it.^¹

This division decides the drug, the length of treatment and the search for a cause. The bladder-infection reflex — short course, done — falls short when it is the kidney.

2. Bladder or kidney? The decisive difference

Three observations usually answer the question: where does it hurt, do you have a fever, how ill do you feel? A bladder infection sits low down and burns; a kidney infection sits round the side at the back and floors you.^¹,²

These are not separate illnesses but stations along the same road. That is why burning when urinating with a fever is something other than burning on its own.

3. Symptoms and warning signs

The symptoms develop over hours to a few days. What is typical is the combination of local pain and a whole-body reaction to infection:^²,³

• Flank pain — at the side below the ribs, often on one side and tender when tapped.
• Fever above 38 °C — frequently with shivering chills.
• Feeling seriously unwell — exhaustion, loss of appetite, nausea through to vomiting.
• Bladder symptoms — they can come first, but in some people they are missing altogether.
• Cloudy urine, sometimes blood in the urine.

In older people the course is often atypical: instead of fever and flank pain there is sudden confusion, falls or an unexplained deterioration.^³

4. Causes and risk groups

Almost always the organism comes from your own bowel. Whether that turns into a kidney infection depends less on the bacterium than on whether urine drains away freely and how well your defences are working.^³,⁴

• Female anatomy: the short urethra makes travelling up easier.
• Obstructions to drainage: kidney stones, narrowings of the ureter or an enlarged prostate dam the urine up — an ideal breeding ground.
• Bladder catheters: a way in for organisms; the risk rises the longer the catheter stays in.
• Diabetes: sugar in the urine, weaker defences and impaired bladder emptying make severe courses more likely.
• And more: pregnancy, a weakened immune system, a bladder infection shortly beforehand.

5. Diagnosis: urine culture, blood tests, ultrasound

Fever plus flank pain already tells you a great deal; laboratory tests and ultrasound confirm the diagnosis. The order matters:

• Urine culture before the first dose of antibiotic: afterwards the organism often can no longer be detected — and the resistance testing is then missing at precisely the moment when the first treatment does not work.^¹
• Urine dipstick: white blood cells, nitrite and blood — quick, but not proof.
• Blood test: inflammatory markers, creatinine and urea; blood cultures if the course is severe.
• Ultrasound of the kidneys and bladder: shows a urinary obstruction, stones, abscesses and residual urine — exactly the things that antibiotics on their own would fail on.
• Further investigation: CT or a urology referral if the fever has not come down after 48 to 72 hours, with recurring episodes, and in every man affected.

Do say whether you have recently taken an antibiotic — that changes which organism is likely and how probable resistance is.

6. Treatment: antibiotics, by mouth or as an infusion

Antibiotic treatment is the core of care here: the point is to stop an infection of the tissue before it moves into the bloodstream. The choice depends on severity, on other conditions, on pregnancy and on the resistance situation — the decision is always made by the treating practice.^¹

On the length of treatment: depending on the drug and the course it is usually around one to two weeks — considerably longer than for an uncomplicated bladder infection, which often manages with one to three days; with complicated courses and in men it can be longer still. Fever and pain usually improve within two to three days; if that does not happen, the findings need to be reassessed.^¹

7. Seeing the course through

The most common mistake with this illness: after three days the fever has gone — and the remaining tablets disappear into a drawer. But being free of fever only means that the bacterial load has fallen, not that the kidney tissue has recovered. Stopping too early risks a relapse with the same bacteria.

• Fixed times instead of “by feel” — most antibiotics need evenly spaced intervals.
• Write the end date down — not “a few more days”, but one specific day. Guidance in the article Taking antibiotics correctly.
• Check for interactions — calcium, magnesium, iron and some acid blockers reduce the absorption of certain antibiotics.
• Do not treat yourself from the cupboard — leftover packs match neither in drug nor in dose and encourage antibiotic resistance.

8. A closer look at the drugs: what works, what does not

Fluoroquinolones: effective, but not everyone’s first choice

Ciprofloxacin and related fluoroquinolones reach very good concentrations in the kidney tissue. At the same time the regulators have restricted their use, because rarely — but sometimes lasting a long time — inflammation of the tendons through to tendon rupture, nerve damage with tingling and numbness, and restlessness, sleep problems or confusion can occur.^⁵

What follows from that is not condemnation but a weighing up: for minor infections fluoroquinolones are no longer to be used. In a kidney infection they remain an expressly intended option — above all in more severe courses, where alternatives are not tolerated, or where the resistance situation leaves no other choice.^¹,⁵

Fosfomycin and nitrofurantoin: good for the bladder, unsuitable for the kidney

Fosfomycin (the familiar single-dose powder) and nitrofurantoin are first-choice agents in uncomplicated bladder infection — precisely because they reach effective levels almost only in the urine and the bladder and put hardly any strain on the rest of the body.

In a kidney infection that advantage turns into its opposite: too little of the drug arrives in the kidney tissue. Nitrofurantoin is therefore expressly unsuitable where the kidney is suspected to be involved, and for fosfomycin this indication is not provided for either.^¹ Anyone who takes the leftover sachet of fosfomycin is in practice not treating this at all — but is losing time.

9. Special situations: pregnancy, men, diabetes

In pregnancy

Here every kidney infection counts as complicated, because it is linked with preterm birth and severe courses. Treatment is often given in hospital and the choice of drug is limited — fluoroquinolones are not used. Bacterial colonisation without symptoms is treated as well, unlike in other situations. Never stop prescribed antibiotics on your own initiative.^¹,³

In men

A urinary tract infection in a man counts as complicated as a matter of principle. Behind it there is often an obstruction to drainage from an enlarged prostate, a stone, or involvement of the prostate — which lengthens treatment and calls for a urological assessment. Recurring infections here are never “just bad luck” but an instruction to look for the cause.^⁴

With diabetes, catheters and stones

People with diabetes have severe courses more often; high blood sugar worsens the body’s defences. With an indwelling catheter, bacterial colonisation is almost the rule — it is treated only if there are symptoms, otherwise you are breeding resistance. With kidney stones and fever there is no time to lose: an infected urinary obstruction is a urological emergency.

10. Everyday life and prevention without the myths

Alongside the antibiotic, the first few days are about three things: fluids, bearable pain and rest. All of them support the treatment — none of them replaces it.

• Drink sensibly: drinking enough keeps the urine moving. Extreme amounts bring no extra benefit and are risky with heart failure or impaired kidney function.
• Painkillers with purpose: paracetamol brings fever and pain down without affecting the blood flow through the kidneys. Ibuprofen works well, but can put a strain on the kidneys with fever, fluid loss and blood pressure or water tablets taken alongside.
• Warmth and taking it easy: a hot water bottle on the flank can soothe; physical exertion belongs on pause.
• Do not forget the review: if fever and pain do not improve within two to three days, go back and be seen again.

Prevention: what is proven and what is not

• Empty your bladder regularly and completely rather than holding on for long stretches, particularly after sex.
• Cranberry products: the evidence is limited and inconsistent; a try with recurring bladder infections is considered reasonable.^²
• D-mannose and herbal preparations: thin evidence — as a replacement for antibiotics they are out of the question.
• What really counts: having obstructions to drainage treated, keeping blood sugar well controlled, avoiding unnecessary catheters, having recurring infections investigated.
• What does not help: wrapping up warm is no substitute for treatment — being cold is not a cause of a bacterial kidney infection.

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