When a Fever Needs a Blood Test

A fever is your body reacting to an infection, and most fevers settle by themselves. But some serious infections start out looking exactly like an ordinary fever, and the only reliable way to tell is a test done in a laboratory, not a guess at home.

Why can't you tell from the symptoms?

Fever, headache, body aches, chills, tiredness, nausea and loss of appetite appear in many different infections. They are shared symptoms, not clues pointing to one disease. WHO notes that malaria symptoms can be mild, especially in people who have had malaria before, which is exactly why early testing matters instead of waiting to see what happens.

Guessing costs time. WHO warns that untreated Plasmodium falciparum malaria can progress to severe illness and death within 24 hours, while treating mild malaria early can stop it becoming severe. Dengue can also turn severe. Typhoid causes a prolonged high fever and can be fatal in severe cases. None of these should be managed on a hunch.

What tests do doctors use?

Different infections need different tests, and a doctor chooses based on your symptoms and where you have been.

• Malaria: WHO recommends that all suspected cases be confirmed using a parasite-based test, either microscopy of a blood sample or a rapid diagnostic test. Treatment should follow a positive test, not a guess.
• Dengue: tests used in the first week of illness include nucleic acid amplification tests, laboratory antibody tests and rapid diagnostic tests.
• Typhoid: WHO says typhoid fever can be confirmed through blood testing.

Tell the doctor when the fever started, whether it comes and goes, what else you feel, and where you have travelled recently.

Why not just take antibiotics?

Antibiotics do nothing against dengue or malaria, which are not bacterial infections. For typhoid, WHO says antimicrobial resistance is common, meaning the usual medicines stop working and treatment becomes more complicated and more expensive. Only a doctor should decide whether an antibiotic is needed, and if you are prescribed a course you must complete all of it, even once you feel better.

One safety point: if dengue is possible, WHO advises against non-steroidal anti-inflammatory painkillers such as ibuprofen and aspirin, because they can increase the risk of bleeding. Ask a doctor or pharmacist which fever medicine is safe for you, and rest and drink plenty of fluids while you wait to be seen.

When to see a doctor

See a doctor and ask about testing if a fever lasts more than a day or two, keeps coming back, or you are getting worse instead of better. Children under 5, pregnant women and people with weakened immunity should be seen early rather than late.

Go to hospital immediately if an adult or older child has any of these:

• Severe stomach pain, or vomiting that will not stop
• Bleeding from the gums or nose, or blood in vomit or stool
• Fast breathing or difficulty breathing
• Confusion, slurred speech, or not making sense
• A fit or seizure, or being very difficult to wake
• Blue, pale or blotchy skin, or skin that is cold and clammy
• A rash that does not fade when you press a glass or finger against it
• Dark or bloody urine, yellowing of the skin or eyes, or unusual bleeding
• Extreme thirst, extreme weakness or restlessness

For a baby or child under 5, go immediately if they are breathing very fast or struggling to breathe, have blue, grey, pale or blotchy skin, lips or tongue, have a rash that does not fade when pressed, have a weak, high-pitched or continuous cry, are not feeding, are difficult to wake or much sleepier than usual, or have not passed urine for 12 hours. A temperature of 38C or more in a baby under 3 months, 39C or more at 3 to 6 months, or below 36C at any age also needs immediate care.

In an emergency, call Rescue 1122 where available, or go to the nearest hospital emergency department. Do not wait for the fever to break first.