A. Probably not. There are many things besides a sinus infection that can cause a runny nose, including a simple cold or allergies.
If your child has a lingering runny nose and it is clear, then your child may have allergies. Other symptoms might include a cough, sore throat, and itchy eyes.
Keep in mind that sinusitis is often overdiagnosed in children. Many parents (and doctors) think that a child has a sinus infection at the first sign of a green or yellow runny nose. Instead, these children often just have a viral upper respiratory tract infection (URI), like the common cold, and do not require treatment. In fact, URIs are reported to be 20-200 times more common than a true bacterial sinus infection.
Symptoms of a URI usually begin with a clear runny nose, which may become green or yellow after 2-3 days. Other symptoms can include a fever, cough, headache and decreased activity, although facial pain and facial swelling is not always present in children. These symptoms usually worsen over 5-7 days and then gradually get better. Because they are caused by viruses, URIs do not respond to antibiotics. Antibiotics won't help your child with a cold get better any faster, and when used unnecessarily, being on antibiotics make it more likely that you will get a secondary infection with a bacteria that has become resistant to antibiotics, maker it much more difficult to treat.
The American Academy of Pediatrics recently released a clinical practice guideline on the Management of Sinusitis that should make it easier to diagnose and treat sinus infections in children.
The AAP recommends that the diagnosis of sinusitis be made when children have a runny nose, postnasal drip, and/or a daytime cough, which may worsen at night, and that these symptoms have lasted for more than 10 to 14 days. Children with a shorter duration of symptoms might still have a sinus infection if the symptoms are severe, including 3-4 days of fever (over 102 degrees F) in a child that appears ill.
It is important to keep in mind that having persistent symptoms, which are lasting 10-14 days, doesn't necessarily always mean a sinus infections though. If the symptoms are slowly improving, then even if they are lasting more than 2 weeks, it is probably still just a viral URI.