Having a fever that lingers for days or even weeks can be distressing and confusing. Many people expect fevers to resolve quickly with rest, fluids, or a short course of treatment. However, when a fever persists, it often prompts doctors to look deeper for possible hidden causes. This article explains the common reasons for prolonged fever, the warning signs that suggest a more serious illness, the tests that may be used, and what you can expect during the medical evaluation process.
Common causes of prolonged fever
A fever lasting more than a week is sometimes called a “prolonged” or “persistent” fever. Doctors may also use the term “fever of unknown origin” (FUO) if it continues for an extended period without a clear explanation. There are many potential causes, and often, the source is eventually found. Some of the most common reasons include:
- Infections: This is the most frequent cause. Persistent fevers may be due to bacterial infections (such as tuberculosis or abscesses), viral infections (like mononucleosis), or sometimes parasitic or fungal diseases.
- Inflammatory conditions: Autoimmune or inflammatory diseases, such as rheumatoid arthritis, lupus, or vasculitis, may provoke ongoing fevers.
- Cancers: Some cancers, particularly lymphomas or leukemias, can present with a persistent fever. This is more common in adults and older individuals.
- Medications: Certain medicines can trigger medication-induced fevers as a side effect, especially antibiotics or anti-seizure medications.
- Other less common causes: These include hormonal problems (such as an overactive thyroid), blood clots, or other rare disorders.
In many cases, a thorough history and examination help narrow down the possible reasons. Sometimes, the cause may remain elusive even after extensive testing, but most persistent fevers are eventually explained.
Warning signs of serious infection
While most prolonged fevers are caused by treatable infections or inflammatory conditions, certain symptoms may signal a more serious or rapidly evolving illness. If you or someone you care for has a persistent fever, watch for the following warning signs:
- Severe headache, neck stiffness, or confusion: These may indicate a brain infection such as meningitis or encephalitis.
- Shortness of breath or chest pain: These could be signs of pneumonia, blood clots in the lungs, or heart involvement.
- Severe abdominal pain or swelling: This may point to infections or abscesses in the abdomen, or inflammation of organs such as the liver or spleen.
- Persistent vomiting, inability to keep fluids down, or signs of dehydration: These symptoms require prompt medical attention.
- Widespread rash, especially if it is purple or does not blanch when pressed: This could be a sign of a serious infection affecting the blood vessels.
- Rapid heart rate, low blood pressure, or confusion: These may signal sepsis, a life-threatening response to infection.
If any of these symptoms occur, seek medical care immediately. Early treatment can be life-saving in serious infections.
Which blood tests or imaging may be used
When a fever lasts longer than expected, doctors often order a series of tests to search for the underlying cause. These may include:
- Routine blood tests: A complete blood count (CBC), markers of inflammation (such as CRP or ESR), and tests of liver and kidney function help provide clues.
- Blood cultures: These are used to check for bacteria in the bloodstream, particularly in patients who may have sepsis or a hidden infection.
- Urine tests: Infections of the urinary tract or kidneys can cause prolonged fever, so urine analysis and culture are often performed.
- Chest X-ray: This helps detect pneumonia, tuberculosis, or other chest infections.
- Ultrasound or CT scans: Imaging of the abdomen or other areas may be needed to look for abscesses, enlarged organs, or tumors.
- Specialised tests: Depending on the findings, tests for specific infections (such as tuberculosis, HIV, or malaria), autoimmune markers, or biopsy of tissues may be considered.
The choice of tests depends on your symptoms, medical history, and findings on physical examination. Not every patient will need all of these investigations.
How doctors narrow down the diagnosis
Persistent fever can challenge even experienced doctors. The process of diagnosis is often stepwise and methodical:
- Detailed history: Your doctor will ask about recent travel, animal exposures, hobbies, medical history, and medications. These details help identify possible sources of infection or inflammation.
- Physical examination: Careful examination may reveal signs such as swollen lymph nodes, rashes, joint swelling, or heart murmurs that provide important clues.
- Initial investigations: Basic blood and urine tests, along with chest X-rays, are often the first step. Results guide further testing.
- Targeted tests: If initial results suggest a specific direction (for example, abnormal liver tests or enlarged lymph nodes), further tests or specialist referrals may follow.
- Observation and repeated assessments: Sometimes, no immediate cause is found. In these cases, doctors may observe your progress, repeat key tests, and watch for new symptoms over time. This approach can be especially important if your general condition is stable.
It is not unusual for the process to take several days or even weeks, especially if the symptoms are mild and you are otherwise well. Your doctor’s goal is to balance the need to find the cause with minimising unnecessary or invasive tests.
When a cause is not found
In a small number of cases, extensive investigations do not reveal a specific cause for the fever. If you are otherwise healthy and your symptoms improve, this may be reassuring and no further intervention is needed. However, ongoing follow-up is often recommended, and any new symptoms should be reported promptly.
What happens after evaluation
Once the cause of a persistent fever is identified, your doctor will discuss the best treatment options. This may include:
- Antibiotics or antivirals: If a bacterial or viral infection is confirmed, appropriate medications will be prescribed.
- Treatment for inflammatory conditions: Autoimmune or inflammatory diseases may require medications to reduce immune system activity, such as steroids or other immunosuppressive medications.
- Management of cancer: If a cancer is diagnosed, you will be referred to specialists for further assessment and treatment planning.
- Stopping or changing medicines: If a medication reaction is suspected, stopping the responsible medication usually leads to improvement.
- Supportive care: Regardless of the cause, managing symptoms such as dehydration, pain, or discomfort is important during recovery.
If no clear cause is found but you remain unwell, your doctor may recommend ongoing monitoring or referral to a specialist for further evaluation. It is important to keep regular follow-up appointments and communicate any changes in your health.
For those interested in understanding more about the laboratory process, you can learn how doctors interpret initial laboratory investigations by reading our guide on what blood tests doctors usually check first when investigating unexplained symptoms.
When to seek further help
If your symptoms worsen, or if you develop any of the warning signs mentioned earlier, seek medical attention promptly. Trust your instincts—if you feel something is seriously wrong, it is always better to err on the side of caution and consult a healthcare professional.
Conclusion
Persistent fever is a common challenge in medical practice. While it can be worrying, most cases are eventually explained and treated successfully. A thorough evaluation, careful monitoring, and open communication with your healthcare team are key to finding answers and ensuring your well-being. If you are experiencing a fever that will not go away, do not hesitate to reach out to your doctor for advice and support.
