Fever Workup: Temperature Chart, CRP, CBC and Next Steps

Fever is a sign, not a diagnosis. In many cases, a short-lived viral infection is the cause and will resolve on its own. Doctors speak of a "fever workup" or "fever evaluation" when they go beyond observation and start a structured diagnostic process to look for the underlying cause.

1. When a fever workup becomes important

Key questions in everyday practice include:

How long has the fever been present – 1–3 days, 4–7 days, or longer?
Is there an obvious focus such as cough, urinary symptoms or abdominal pain?
Are there risk factors such as advanced age, chronic disease or immunosuppression?
Does this meet criteria for fever of unknown origin (FUO)?

Depending on the situation, watchful waiting and self-care may be sufficient at first. Persistent fever, fever without typical cold symptoms, or warning signs such as shortness of breath, confusion, severe weakness or rash warrant medical evaluation.

2. Temperature measurement and fever temperature chart

A temperature chart is more than a notebook entry. It helps to document how high the fever is, how it fluctuates over the course of the day, and how it responds to antipyretics.

2.1 How to measure temperature correctly

For a meaningful fever curve, the following points are important:

Site: Rectal measurement is most accurate in small children; ear, forehead or oral thermometers are frequently used in older children and adults.
Device: Use a digital thermometer in good working order and follow the instructions for use.
Frequency: During acute illness, 2–4 measurements per day are often sufficient.
Influencing factors: Avoid hot or cold drinks and smoking for 15–30 minutes before oral measurement; sweat can affect forehead readings.

2.2 Creating a temperature chart

A fever temperature chart is usually a simple table recording time, temperature, medication (for example paracetamol or ibuprofen) and special features such as chills, rash or vomiting.

Clinicians pay attention to patterns such as persistent fever, intermittent fever or fever spikes at night. In combination with laboratory results this can offer clues to the underlying cause.

3. CRP in fever: inflammation markers in the blood

Two commonly used inflammation markers are C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR). Both rise when there is inflammation in the body. CRP usually changes more rapidly than ESR.

3.1 What CRP can and cannot tell you

CRP is a non-specific marker – it signals that inflammation is present but does not show where it is located.
High CRP levels can occur with bacterial infections, but also with severe viral infections, tissue injury, autoimmune disease or after surgery.
A normal CRP does not completely rule out serious illness, especially early in the course of an infection.
When fever lasts for several days, a falling CRP can suggest improvement, while a rising CRP may point to ongoing or worsening inflammation.

Many guidelines prefer CRP over ESR because of its faster response. In some hospital settings procalcitonin is added as an advanced marker to help assess the likelihood of severe bacterial infection, but it is not used routinely in outpatient care.

4. CBC and differential: typical patterns in infection

A complete blood count (CBC or FBC) is a core part of the fever workup. It includes red blood cells, white blood cells (WBCs) and platelets. In the white blood cell differential, the different types of WBCs are counted separately.

Common CBC patterns seen in infections include:

Neutrophilia (high neutrophil count) – common in many bacterial infections.
Lymphocytosis (high lymphocyte count) – often seen in viral infections.
Neutropenia (low neutrophil count) – can occur with some viral infections, medication effects or severe bacterial disease.
Thrombocytopenia (low platelet count) – may be present in certain viral infections and serious systemic illnesses.

Doctors interpret the CBC together with symptoms, examination findings, CRP and the temperature chart.

"No single lab value decides the case. It is the overall picture – symptoms, examination, temperature curve and repeated lab trends – that guides decisions."

5. Typical next steps after CRP and CBC

5.1 Urine testing

Urine testing is particularly important when fever is accompanied by urinary symptoms such as burning, frequency or flank pain, or in small children and older adults where urinary tract infections may be less obvious.

5.2 Blood cultures

Blood cultures are usually taken when there is concern for bloodstream infection or sepsis – for example with high fever and chills, low blood pressure, rapid breathing or other signs of severe illness.

5.3 Imaging and additional labs

Depending on symptoms, further tests may include:

Chest X-ray when fever is combined with cough, chest pain or shortness of breath.
Ultrasound (for example of the abdomen, kidneys or heart) when the focus of infection is unclear.
Extended laboratory testing such as liver and kidney function, electrolytes or lactate in more severe illness.
Specific tests for tropical infections, autoimmune disease or malignancy when clinically suspected.

6. Duration-based pathways: 1–3 days, 4–7 days, longer

6.1 Fever for 1–3 days

In otherwise healthy older children and adults, fever for 1–3 days is often due to self-limiting viral infections. Doctors may initially recommend rest, fluids and, if needed, antipyretics while monitoring the course.

6.2 Fever for 4–7 days

Fever lasting 4–7 days or worsening general condition usually prompts an in-person evaluation with a basic lab panel including CBC and CRP, and sometimes additional tests depending on symptoms.

6.3 Prolonged or recurrent fever (FUO)

Fever of unknown origin is generally defined as fever persisting beyond the time expected for common self-limited illnesses, despite an initial negative workup. At this stage, stepwise diagnostic algorithms are used.

7. High-risk groups and emergency warning signs

Children, older adults, pregnant people, patients with weakened immune systems and those who have recently undergone major surgery are more vulnerable to complications of fever.

Emergency warning signs that warrant urgent care can include:

Difficulty breathing or shortness of breath
Persistent chest pain or severe abdominal pain
Confusion, extreme drowsiness or seizures
Very low blood pressure, cold or mottled skin, feeling severely unwell
Signs of dehydration such as very little urine output, dry mouth and dizziness
Spots or rash that do not fade when pressed, especially together with fever

If any of these occur, people should seek emergency medical care, regardless of what previous lab results have shown.

8. Preparing for your appointment

Patients can support the fever workup by bringing structured information to their appointment:

a temperature chart with date, time, values and fever-reducing medication
an up-to-date medication list including over-the-counter and herbal remedies
details on recent travel, animal exposures, insect bites or specific contacts
previous discharge summaries, lab reports and vaccination records

9. FAQ: common questions about fever workup

What does a high CRP level mean with fever?

A high CRP level indicates a strong inflammatory response in the body. Bacterial infection is one possible cause, but severe viral infections, autoimmune disease and tissue injury can also raise CRP.

Can fever still be viral if CRP is high?

Yes. CRP does not reliably distinguish between bacterial and viral infection. Severe viral illnesses can also produce high CRP values, so treatment decisions are never based on CRP alone.

How often should I check my temperature?

During acute illness, several measurements per day are often enough to see the course of the fever. Constantly checking temperature without a clear medical reason usually does not help.

When are blood cultures needed?

Blood cultures are generally obtained when there is concern for sepsis or bloodstream infection – for example with high fever and chills, low blood pressure or organ dysfunction.

What if CBC is normal but fever persists?

A normal CBC does not exclude significant disease. Persistent or recurrent fever should be reassessed by a clinician.

Disclaimer: This article is for general information only and does not substitute for professional medical advice, diagnosis or treatment.