A kidney infection, known medically as pyelonephritis, is a serious condition that occurs when bacteria travel from the bladder into the upper urinary tract. Unlike a localised bladder infection, a kidney infection is a systemic illness that can lead to permanent organ damage or life-threatening complications if not treated promptly. Recognizing symptoms like flank pain, high fever, and shivering is critical for distinguishing a kidney infection from a simple bladder infection.
What We will cover in this Article
- The primary systemic symptoms of a kidney infection
- Identifying the specific location of kidney-related pain
- Recognizing the ‘red flag’ signs that indicate medical urgency
- How symptoms in children and the elderly may differ
- A comparison table of lower versus upper urinary tract symptoms
- Emergency guidance for severe clinical presentations
Systemic Symptoms and High Fever
One of the most significant signs that a urinary infection has reached the kidneys is the onset of systemic symptoms. While a bladder infection usually stays localised, a kidney infection often makes the entire body feel ill. A high temperature, typically above 38°C, is one of the most common indicators. This fever is frequently accompanied by ‘rigors’ intense, uncontrollable shivering and shaking.
Because the kidneys are in close contact with the bloodstream, the body initiates a vigorous inflammatory response to fight the bacteria. This often results in a profound sense of fatigue, generalized body aches, and a feeling of being ‘wiped out,’ similar to the symptoms of a severe flu.
- High Fever: A sudden rise in temperature, often exceeding 38°C.
- Chills and Rigors: Shivering and feeling cold even when the body is hot to the touch.
- Nausea and Vomiting: Many patients feel sick or find it impossible to keep fluids down.
- Lethargy: Significant weakness and a desire to stay in bed.
Localised Pain: The Flank and Back
The location of pain is the most helpful physical clue in identifying a kidney infection. While a bladder infection causes pain in the lower pelvis, a kidney infection causes pain in the ‘flank’. The flank is the area on the side of the torso, between the bottom of the ribcage and the top of the hip.
This pain is usually felt on one side, depending on which kidney is infected, though it can occur on both sides. It is often described as a deep, throbbing ache that can radiate toward the groin. Clinicians often look for tenderness in the ‘costovertebral angle’ (the area on your back where the ribs meet the spine) as a definitive sign of renal inflammation.
- Flank Pain: Deep-seated pain in the side of the abdomen or mid-back.
- Tenderness: The area may feel extremely sensitive even to a light touch or tap.
- Radiating Pain: The discomfort may move from the back down toward the lower abdomen or groin.
Symptoms in Specific Groups
It is important to note that a kidney infection may not always present with the classic ‘triad’ of fever, flank pain, and urinary stinging. In certain populations, the symptoms can be much more subtle or atypical, which can lead to delays in diagnosis.
In the elderly, a kidney infection may not cause a fever at all. Instead, it might manifest as sudden confusion, agitation, or a change in personality a state known as delirium. In infants and very young children, the only sign might be poor feeding, irritability, or an unexplained high temperature.
Comparison: Bladder vs. Kidney Symptoms
| Feature | Bladder Infection (Cystitis) | Kidney Infection (Pyelonephritis) |
| Pain Site | Lower pelvis / Above pubic bone | Side / Mid-back / Flank |
| Temperature | Usually normal | High fever (>38°C) |
| Systemic Illness | No | Yes (Nausea, vomiting, rigors) |
| Urination | Stinging and frequency | Stinging and frequency |
| Urgency of Care | Primary Care (GP/Pharmacy) | Urgent Medical Evaluation |
To Summarise
Symptoms that suggest a kidney infection are generally more severe than those of a standard UTI. The presence of high fever, shivering, vomiting, and pain in the side or back are all strong indicators that the infection has moved beyond the bladder. Because of the risk of urosepsis and permanent kidney scarring, these symptoms should never be ignored and require immediate assessment by a healthcare professional.
If you experience severe, sudden, or worsening symptoms, such as high fever, uncontrollable vomiting, or intense pain in your side or back, call 999 immediately.
Is back pain always a sign of a kidney infection?
No, back pain is often muscular. However, if the pain is in the side/mid-back and occurs alongside fever or stinging when you pee, it is a significant warning sign.
Can a kidney infection cause confusion?
Yes, especially in older adults. A systemic infection can cause delirium, which may be the only obvious symptom in elderly patients.
Does a kidney infection always start with a bladder infection?
In most cases, yes. Bacteria usually travel from the bladder upwards. However, bacteria can occasionally reach the kidneys through the bloodstream from an infection elsewhere.
Can I treat a kidney infection at home with fluids?
No. While staying hydrated is important, a kidney infection is a serious bacterial illness that requires prescription antibiotics to prevent permanent damage.
Why does a kidney infection cause vomiting?
The systemic inflammatory response affects the whole body, including the digestive system, and can be a sign that the infection is becoming severe.
Authority snapshot
This article was reviewed by Dr. Stefan Petrov, a UK-trained physician with an MBBS and extensive experience in both general medicine and emergency care. Dr. Petrov has managed numerous cases of pyelonephritis and urosepsis in hospital wards and intensive care settings. This guide follows NHS and NICE safety protocols to ensure the public can identify the critical differences between localised and systemic urinary infections.
