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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 or nasal culture, or blood test to rule out a secondary infection. If that's the case, what are those likely infection?

These article about Common cold and diagnosing (2) have given ideas on doctor asking for test, but I would like a simple explainable answer from this community.

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    I had both the flu, cold and fever and never had a blood test conducted on me. This might depend on country regulations, culture and training of the doctor. All I can tell you is why the doctor might have done it. There is always the option to ask them straightforwardly – Narusan Aug 11 '17 at 7:14
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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.

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    +1 Thanks for fixing my mistakes :D. It is very uncommon here in Germany to do blood tests when one is suffering from a cold (if at all X-ray for pneumonia) so I just drew my conclusion from the nytimes article. – Narusan Aug 11 '17 at 7:16

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