Urinary Tract Infection and Pyelonephritis: Infection from Bladder to Kidney

A urinary tract infection can affect the bladder, the kidneys or another part of the urinary system. Lower UTI usually means infection in the bladder, while pyelonephritis is an infection involving one or both kidneys. The distinction matters because kidney infection carries a greater risk of serious illness and complications.

The urinary tract and where infection occurs The urinary tract includes the kidneys, ureters, bladder and urethra. The kidneys make urine, which travels through the ureters to the bladder. Urine then leaves through the urethra. Most urinary tract infections are caused by bacteria from the bowel entering the urethra. The bacteria may multiply in the bladder or travel further upwards. The term UTI describes several possible sites of infection. It does not tell you automatically how severe the illness is or which treatment is appropriate. Lower UTI and cystitis A lower UTI usually affects the bladder and is often called cystitis. Urethritis affects the urethra and can produce similar symptoms, although it may have different causes. Typical lower urinary symptoms include pain or burning during urination, increased frequency, new urgency and passing urine during the night. Lower abdominal discomfort, cloudy urine or visible blood may also occur. Dark or strong smelling urine alone does not establish a UTI. It can occur when urine is concentrated because someone has not drunk enough fluid. Lower UTI usually does not cause marked systemic illness. Fever, shivering, pain in the back below the ribs, vomiting or significant deterioration raises concern about kidney involvement or another serious condition. Pyelonephritis and kidney infection Acute pyelonephritis is an infection of one or both kidneys. It is usually caused by bacteria travelling upwards from the bladder. A kidney infection can sometimes occur without clear preceding bladder symptoms. The person may mainly experience fever, shivering, nausea, vomiting or pain in the side or back below the ribs. Pyelonephritis is more serious than uncomplicated cystitis. It can impair kidney function, spread into the bloodstream or contribute to sepsis. Pregnancy, urinary obstruction, kidney stones, structural urinary abnormalities, diabetes and impaired immune function can increase the risk of complications. These factors do not prove that pyelonephritis is present. How bacteria reach the urinary tract Bowel bacteria commonly live harmlessly around the skin near the urethra. They can enter the urethra and ascend into the bladder. A shorter urethra makes ascending infection more common in people with a female urinary system. Sexual activity, pregnancy and changes associated with menopause can alter risk. Urine normally flows from the kidneys towards the bladder and out of the body. Anything that obstructs flow or prevents complete bladder emptying can make infection more likely or harder to clear. Examples include urinary stones, an enlarged prostate, neurological bladder problems and some structural abnormalities. A urinary catheter also creates a route for bacteria and requires a separate diagnostic and treatment approach. Symptoms do not confirm the diagnosis alone Painful urination, urgency and frequency make lower UTI more likely, but none is unique to infection. Vaginal conditions, sexually transmitted infections, urethral irritation, bladder pain syndrome and prostate disease can produce overlapping symptoms. Fever and flank pain raise concern about pyelonephritis, but they can also have other causes. A clinician considers the whole pattern rather than relying on one symptom. A urine dipstick can sometimes help assess non pregnant women under 65 when the symptom pattern is uncertain. It does not diagnose the site or severity of infection by itself. Urine culture can identify bacteria and test which antibiotics are likely to work. A negative or mixed result still needs interpretation alongside symptoms, collection quality and previous treatment. Asymptomatic bacteriuria is different from UTI Asymptomatic bacteriuria means bacteria are found in urine without urinary symptoms or other signs of infection. It becomes more common with age and is particularly common among frail care home residents. In men and non pregnant women, asymptomatic bacteriuria is not routinely treated with antibiotics. Treatment usually provides no benefit and can cause adverse effects or increase antibiotic resistance. Pregnancy is an important exception. Bacteriuria without symptoms is treated during pregnancy because it increases the risk of pyelonephritis and premature delivery. A positive urine result therefore means different things in different clinical settings. It should not be interpreted without knowing whether symptoms are present and whether the person is pregnant. Older adults need careful assessment Older adults can develop genuine lower UTI or pyelonephritis. They may have local urinary symptoms, fever, rigors, new incontinence or systemic deterioration. However, confusion alone does not prove that the urinary tract is the source. Delirium may result from dehydration, constipation, medicines, pain, another infection or several other acute illnesses. Asymptomatic bacteriuria is common in older people. For this reason, urine dipsticks are unreliable for diagnosing UTI in adults over 65 and should not be used as a stand alone explanation for confusion. Clinicians look for new urinary symptoms, fever, rigors or other supporting evidence. They also assess alternative causes of delirium and do not assume that a positive urine test explains the person's condition. Pregnancy changes the level of concern Pregnancy increases the clinical importance of urinary infection. A suspected lower UTI is treated promptly, and a urine sample is normally collected for culture before antibiotics are started. Antibiotic choice must take pregnancy, gestational stage, allergies and culture results into account. Medicines suitable for one person or one stage of pregnancy may not be suitable for another. Suspected pyelonephritis during pregnancy needs urgent clinical assessment. NICE advises considering hospital referral or specialist advice because of the increased risk of complications, including preterm labour. Recurrent UTI during pregnancy also needs specialist input. Preventive medicines should not be selected without obstetric or other relevant specialist advice. Children, men and people with catheters UTI in babies and children can present differently from adult cystitis. Fever, vomiting, poor feeding, irritability, reduced activity or new wetting may be more prominent than painful urination. A urine sample is usually obtained before antibiotics when this can be done without delaying urgent treatment. Recurrent or upper UTI in children may require imaging or paediatric assessment. Men and people with a male genitourinary system are offered prompt antibiotics for suspected lower UTI, with urine sent for culture. Prostate involvement, obstruction or another underlying cause may need consideration. Catheter associated UTI is not diagnosed from cloudy urine or bacteriuria alone. Long term catheters commonly become colonised with bacteria, so symptoms and systemic findings remain essential. How clinicians investigate possible UTI Assessment begins with the symptom pattern, duration, severity and risk factors. Questions cover fever, shivering, flank pain, vomiting, pregnancy, urinary flow, previous infections, recent antibiotics and structural urinary problems. The clinician may examine the abdomen and back and record temperature, pulse, blood pressure, breathing rate and alertness. Signs of sepsis or inability to maintain hydration change the urgency. Not every uncomplicated lower UTI requires a urine culture. Culture is particularly important in pregnancy, men, children, suspected pyelonephritis, recurrent infection, treatment failure and situations where resistant bacteria are more likely. For acute pyelonephritis in adults, NICE recommends obtaining a midstream urine sample before antibiotics and sending it for culture and susceptibility testing. Treatment should not be dangerously delayed in a severely unwell person. Blood tests may assess kidney function, inflammation and complications. Imaging may be considered when obstruction, stones, an abscess, structural disease or failure to improve is suspected. Treatment principles Some non pregnant women with mild lower UTI may receive an immediate antibiotic or a back up prescription, depending on symptom severity and complication risk. Men, pregnant people and children are offered immediate antibiotic treatment. Pyelonephritis requires antibiotics. Oral treatment may be appropriate when the person can take oral medicines and is not severely unwell. Hospital treatment may be needed for sepsis, severe illness, significant dehydration, persistent vomiting or inability to take oral treatment. Pregnancy and increased complication risk also lower the threshold for specialist advice or hospital assessment. The antibiotic is selected according to the infection site, pregnancy status, kidney function, previous cultures, recent antibiotic exposure and local resistance. A drug used for uncomplicated cystitis is not automatically appropriate for kidney infection. Culture results allow clinicians to review treatment. They may change to an antibiotic that better matches the organism or use a narrower spectrum option when safe. Symptoms should begin improving after treatment starts. Worsening at any time, becoming systemically very unwell or no initial improvement within 48 hours requires reassessment. Recurrent UTI In adults, recurrent UTI means at least two episodes within six months or three within twelve months. It can include recurrent lower UTI or repeated pyelonephritis. A recurrence may be a relapse involving the same bacterial strain or a new infection caused by another organism. Repeated symptoms do not always mean repeated bacterial infection, so the diagnosis and alternative causes need review. Current infection should be treated before preventive treatment is considered. Previous urine cultures, triggers, menopause related changes and possible urinary abnormalities help guide the plan. Possible preventive options include vaginal oestrogen for suitable people around or after menopause, a single antibiotic dose linked to an identifiable trigger, methenamine hippurate in selected non pregnant people, or daily antibiotic prophylaxis. Each option has limitations and requires periodic review. Specialist advice is recommended for recurrent upper UTI, recurrent lower UTI with an unknown cause, pregnancy, children, and people with a male genitourinary system. Investigation may look for stones, obstruction, incomplete emptying or another structural problem. Recovery and preventing complications Uncomplicated lower UTI often begins improving within a few days. Pyelonephritis may take longer, and fatigue or discomfort can persist after fever settles. Drinking enough fluid to avoid dehydration is sensible, but forcing excessive fluid does not replace treatment. Pain relief may be used when suitable for the individual. Regularly delaying urination and incomplete bladder emptying may contribute to infection in some people. Addressing constipation, urinary obstruction, catheter problems or mobility barriers may be relevant to prevention. Cranberry products, D mannose and probiotics are sometimes used for recurrent UTI, but evidence is limited or uncertain. They should not delay assessment or replace prescribed treatment. What this lesson should not be used for This lesson explains the difference between bladder and kidney infection. It cannot diagnose UTI from urine colour, confusion, a dipstick result or one symptom. Do not use it to choose or reuse antibiotics. Seek professional assessment when symptoms suggest kidney infection, the person belongs to a higher risk group, or the condition is worsening.

Lower UTI usually affects the bladder, while pyelonephritis involves one or both kidneys and carries a greater risk of sepsis and complications. Diagnosis depends on symptoms, risk factors and appropriately interpreted urine testing. Pregnancy, older age, recurrent infection, childhood, male urinary anatomy and urinary catheters each require a different clinical approach.