: Reason behind testing blood when someone is suffering from Common cold and fever? What does testing blood helps identifying in case of Common cold or fever? This article about Common cold states, a doctor may do a throat

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There are differences between individual clinicians' ways of approaching things, and regional ones as well, so I can't speak for every clinician.

But respiratory infections are one of the most common reasons why someone comes into primary care or urgent care, and there are common approaches.

When someone comes in with cold symptoms, many factors that are taken
into consideration. Based on those factors, clinician might order lab tests or might not.

The reason is that the differential diagnosis of respiratory infections is broad.

Many viruses like the cold and flu
Pneumonia (primary bacterial or superimposed after viral infection)
Bronchitis
Bacteria like strep throat or sinus infection
Allergies
Chemical irritation

Pneumonia and sinus infections (and very rarely blood infections or meningitis) are possible after a viral infection because the mucus from the cold sitting in the sinuses/lungs are a place bacteria like to grow in. Those are usually the only times antibiotics are worthwhile.

You mentioned symptoms "I have a cold." Those aren't symptoms. Symptoms are

cough (dry or productive of sputum)
runny nose
sore throat
hoarse voice
headache
body aches
fevers or chills
fatigue
bleeding
nasal congestion
weakness
lightheadedness
nausea/vomiting

The constellation of symptoms and timeline are the primary things that tell us WHAT the infection is. Adding vital signs (temp, blood pressure, pulse) and a physical exam, often we don't even need labs/tests.

But when they do, labs/tests they might get are:

Nasopharyngeal swab if they suspect flu and want to be certain, or a viral panel for rhinovirus etc
Throat swab / culture for suspicion of strep throat
Chest XRay if there's evidence of pneumonia (which is not as common as you think just because you cough a lot)
Complete blood count to check your white blood cells. This is usually high when you have any kind of infection, but the differential can clue you in to whether it's bacterial or viral (another topic of discussion). This isn't always done unless you are concerned for a serious infection like severe pneumonia, as the fact the WBCs are high doesn't add much to the picture.
If there's an underlying disease making the respiratory infection more risky, like COPD, asthma, lung diseases, etc, they may want to check other blood tests.
If highly suspected maybe blood tests to check for uncommon things like genetic conditions, HIV. Or mono.

Usually the only reason they would do a test is if it would change management of the condition. Often, there's no point determining which virus it is, you'd manage it the same, except early intervention in flu, severe infections, or young kids. An elevated WBC doesn't tell you much except "likely infection" which you already know by the clinical symptoms. Etc.

Note, ESR and CRP have low role in respiratory infections generally, unless we're looking at sepsis from pneumonia/sinus infection, and then you'd usually add lactate and trend those.