How do doctors know when an infection is viral or bacterial?
Wednesday, April 21st, 2010 at
12:16 pm
Whenever I go see a doctor for a sore throat, they generally know if I have a bacterial infection, which is then treated with antibiotics. How do they know the infection is not caused by virus?
Filed under: Bacterial Vaginosis
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Bacterial infections are usually accompanied by pus production, while viral infections are not. Collecting a sample and culturing it is necessary to make a definitive diagnosis.
Temperature patterns, exam (how the throat actually looks) lymphadenopathy and the rest of the physical signs and symptoms, and blood work (lymphocytes elevated in viral, neutrophils or their precursors elevated in bacterial.)
And years of med school and residency.
1. Bacteria can be easily seen under the microscope
because they are larger in size. Therefore, they
can tell that it may be a bacteria instead of a virus,
because viruses are so small and need very high
power microscopes and other machines to see them.
2. Bacteria can reproduce themselves easily.
Virus need a living host to reproduce themselves.
They usually use the patients own cells, with
the nucleus of the cells own DNA/RNA to
reproduce.
3. Bacteria usually respond to antibiotics, very
few viruses do. They need drugs like Interferon
and others to be destroyed or "combinations" of
very strong antibiotics to try to do this.
4. Viruses, when the reproduce, they can change
their structure just slightly…known as a mutation…
in which drugs then may not have any effect whatsoever
on them. Because of the mutation, they are harder
to destroy…since more drugs may be needed to
be developed to do this.
There are other things that they look for in testing
that give them more of an idea whether it is a
virus or bacterial. I hope this information has been
of some help.
they know by classifying the signs and symptoms.
Usually, a specific disease may be caused by hundreds of bacteria and virus, but a certain sign or symptom can be indicative of the presence of a particular bacteria or virus. and this sign or symptom can also indicate whether it is a viral or bacterial infection.
Doctors learn this in premed, med school and during the years of their practice. Basically, they’ve memorize most of the bacteria/virus and the respective diseases they can cause.
If they are unsure, they ask for a culture and sensitivity test. the culture part is for knowing what microorganism caused the disease and the sensitivity part is to determine what medication is appropriate to kill the microorganism
With throats, it’s never certain, but the "Centor criteria" are a reasonable guide for clinical use.
Most cases of sore throat are viral. Nowadays physicians can do a rapid strep test in the office to rule in or out a streptococcal infection. The test takes less than 5 minutes. Yes, there are other bacteria that can cause a sore throat, but they can be only diagnosed with certainty by a throat culture which takes 24 – 48 hours, and relatively speaking, they are less common.
There is no way to be sure for most infections, and cultures (trying ot grow the bacteria) are only done for infections that do not seem to resolve, since they are time consuming and expensive. Very few doctors have the ability to culture and then look at the bacteria under microscope in the office, and even if they did, a culture could take a couple days or even a week to yield results.
Basically, the way we do it is by the process of elimination. Certain symptoms are encountered more with viral infections and others are more with bacterial. For a common example, consider a cold. You have nasal stuffiness, a runny nose, maybe some post-nasal drip. Colds are caused by a virus. Now add a symptom of severe congestion and maybe pain on the sides of the nose, and you have a sinus infection, which is bacterial. So its basically, doctors learn in medical school and residency what symptoms go with which diseases, and which organisms can cause said diseases. Then you use the process of elimination on the patient and come down with a likely culprit and treat as such.
Lab tests like blood counts are rarely used for routine infections, and they along with cultures are usually used for more serious infections, like someone presenting with meningitis symptoms, where knowing for sure if it is bacterial or viral can mean the difference between life and death fairly quickly.
To clear up some other points brought up on here, anti-viral medications are not used much to treat most common viral infections, as they are rarely cost effective, since the body can clear the infection almost as quick without any medication.