Sore Throat: Causes, Tests & When to Worry

What Causes a Sore Throat?

Most sore throats are caused by viral infections like the common cold or flu and resolve on their own within a week. A smaller number are caused by bacteria, most notably strep throat, which needs antibiotics.

Common Causes

• Viral infections (cold, flu, mononucleosis)
• Strep throat (bacterial)
• Allergies and postnasal drip
• Acid reflux (GERD)
• Dry air or irritants (smoke, pollution)
• Tonsillitis

Strep Throat vs. Viral: Key Differences

Tests Your Doctor May Order

Frequently Asked Questions

References

Sources cited on this page. PubMed links open the original abstract.

1. McIsaac WJ, Goel V, To T, Low DE. Empirical validation of guidelines for the management of pharyngitis in children and adults. JAMA. 2004;291(13):1587–1595. PMID 15069046 · doi:10.1001/jama.291.13.1587

When a Sore Throat Needs Blood Tests

Most sore throats are caused by viral upper respiratory tract infections – rhinovirus, coronavirus, adenovirus, or influenza – and resolve within seven to ten days without specific treatment or blood investigation. Blood tests are indicated when the clinical picture suggests bacterial infection requiring antibiotic treatment, complications from untreated infection, or an underlying systemic condition presenting as pharyngitis.¹

Group A Streptococcus (GAS) pharyngitis accounts for 15–30% of sore throats in children and 5–15% in adults and is the target of antibiotic treatment to prevent complications including acute rheumatic fever and peritonsillar abscess. Clinical scoring tools – the Centor and McIsaac criteria – help stratify risk without blood tests, using features such as fever, tonsillar exudate, anterior cervical lymphadenopathy, and absence of cough. A Centor score of 1 or below has a negative predictive value of approximately 80% for GAS, making testing unnecessary in low-risk presentations. Rapid antigen detection tests (RADTs) or throat swabs provide microbiological confirmation without blood testing in most cases.

Infectious Mononucleosis: When the Sore Throat Is Something More

Infectious mononucleosis (glandular fever), caused by Epstein-Barr virus (EBV), is the classic "sore throat that isn't getting better" in adolescents and young adults. The full blood count reveals the characteristic pattern: lymphocytosis with more than 10% atypical (reactive) lymphocytes – large, irregular cells visible on a blood film – together with mild thrombocytopenia and, in approximately 80% of cases, elevated liver transaminases (ALT, AST) from EBV hepatitis.²

The Paul-Bunnell (monospot) test detects heterophile antibodies and is positive in approximately 85% of cases in adolescents and adults, but less sensitive in young children and in the first week of illness. When the monospot is negative and EBV is still suspected, EBV-specific serology (VCA IgM, VCA IgG, EBNA IgG) confirms the diagnosis and can distinguish acute primary infection from past infection. Splenomegaly occurs in up to 50% of cases – an important clinical finding because splenic rupture, though rare, is a serious complication of contact sports; patients should be advised to avoid contact sports and heavy lifting until confirmed to be in full recovery.

Complications and Systemic Conditions That Present as Sore Throat

A persistently sore throat with constitutional features – weight loss, drenching night sweats, or painless lymphadenopathy – requires urgent investigation to exclude lymphoma. Full blood count with film, LDH, uric acid, and urgent haematology referral are appropriate when these features are present. Agranulocytosis – a dangerous drop in neutrophils – presents as a severe sore throat in patients on carbimazole, clozapine, or certain other medications; a full blood count showing neutrophils below 0.5 × 10⁹/L is a medical emergency requiring immediate drug cessation and haematology input.³ Leukaemia can also first present as a sore throat with tonsillar enlargement; a full blood count showing blast cells, very high white cell count, anaemia, and thrombocytopenia in this context demands same-day haematology assessment.

Why most sore throats do not need antibiotics

The large majority of sore throats are viral, and antibiotics neither shorten them nor prevent complications. The clinical difficulty is that a viral throat and a streptococcal one can look similar, which is why scoring systems exist. The Centor criteria, and the modified FeverPAIN score used widely in the UK, combine features such as fever, absence of cough, tender neck glands and pus on the tonsils to estimate the chance of a bacterial cause and guide whether to test or treat.

Even with a confirmed strep throat, the benefit of antibiotics is modest for symptoms, shortening illness by around a day. The stronger argument for treating is reducing onward spread and, historically, preventing rheumatic fever, which remains a meaningful concern in some populations and is rare in others.

Sore throats that are not infections

• Acid reflux often presents as a persistent morning sore throat with hoarseness and throat clearing, without fever.
• Post-nasal drip from allergy or sinus disease irritates the throat continuously and improves when the underlying cause is treated.
• Dry air, mouth breathing and voice overuse, common in singers, teachers and anyone recovering from a blocked nose.
• Smoking and vaping, and occupational exposure to dust or fumes.
• Medication effects: inhaled steroids can cause thrush, and some drugs reduce saliva and leave the throat persistently raw.

When a sore throat is an emergency

A few presentations need immediate assessment rather than a swab. Difficulty breathing, drooling because swallowing is impossible, a muffled or hot-potato voice, severe one-sided pain with trismus, or a swollen neck with high fever can indicate abscess or airway compromise. So can a sore throat with a widespread rash, or one in someone taking carbimazole, methotrexate, clozapine or chemotherapy, where a very low white cell count is possible and needs same-day blood testing.

What helps while it settles

Regular paracetamol or ibuprofen, taken at proper doses rather than sporadically, does more for pain than most throat preparations. Anaesthetic lozenges and sprays give short-lived relief. Warm or cold fluids, whichever suits, keep the throat comfortable and maintain hydration. Honey has reasonable evidence for cough and throat discomfort in adults and children over one year, but must not be given to infants under twelve months.