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Strep A testing for sore throat – the facts

  • by
Ross Ferguson

WITH all the media coverage and polarised opinion within pharmacy, I was interested in the numbers that related to Strep A testing in pharmacy. Here is what I found out.

We know that the annual incidence of sore throat that results in a GP visit in the UK is 58.3 per 1,000 population [1]. So, in England, with a population of 53 million people, we can expect around 3 million cases of sore throat to present in GP practices every year.

Viral infection is responsible for 85-90% [2] of cases of acute sore throat, so antibiotics are of no use. The most important cause of bacterial sore throat is group A β-haemolytic streptococcus (GABHS), which causes 10% of sore throats — 300,000* GP visits per year.

We also know that sore throat symptoms resolve within 3 days in 40% of people, and within 1 week in 85% of people, irrespective of whether or not the sore throat is due to a streptococcal infection.

So, after one week, 45,000 people with sore throat caused by Strep A will still have symptoms. Distributed amongst England’s 11,600 pharmacies, that’s around 4 people, per year, per pharmacy with sore throat caused by Strep A.

Of course we can’t tell which of the people with Strep A will be better after one week, and we know that antibiotics do protect against the following possible complications [2]:

  • Acute rheumatic fever (number needed to treat [NNT] = 4,000)
  • Subsequent acute otitis media (NNT = 29)
  • Subsequent acute sinusitis (NNT = 50)
  • Subsequent peritonsillar abscess (NNT = 27).

The Centor clinical prediction score can be used to help with a diagnosis of GABHS. The four criteria used are [3]:

  • Presence of tonsillar exudate.
  • Presence of tender anterior cervical lymphadenopathy or lymphadenitis.
  • History of fever.
  • Absence of cough.

The likelihood of GABHS infection increases as the score increases (one for each symptom). A score of 3 or 4 suggests that the person may have GABHS (40–60% chance) and may benefit from antibiotic treatment [3].

If three or four of these signs are absent, it suggests that the person is unlikely to have an infection (80% chance), and antibiotics treatment is unlikely to be necessary [3].

The Centor clinical prediction score should be used to assist the decision on whether to prescribe an antibiotic, but cannot be relied upon for a precise diagnosis [1]. It can be used to identify those who may have GABHS (as in the recently publicised test and treat scheme), who may then receive a Strep A throat test.

Strep A throat test
The OSOM Strep A test which was used in the trial, has a sensitivity of 96%, and a specificity of 98% and results are available within 5 minutes [4].

The sensitivity (true positive) and specificity (true negative) provides information about how accurate it is at identifying those who have GABHS and those who don’t.

In this case, out of 100 people who have GABHS, 4 will not get a positive result, and out of 100 people who don’t have GABHS, 2 will not get a negative result. As with any screening test that is not 100% accurate, this may result in people with the condition not being treated, and those that don’t have it being inappropriately treated.

We also know that throat swab culture cannot differentiate between the streptococcal carrier state and invasive infection [1]. Interestingly, 30% of people with GABHS may be carriers, but will not have any symptoms and cannot infect others [2].

SIGN recommendations [1]

  • Throat swabs should not be carried out routinely in primary care management of sore throat.
  • Throat swabs may be used to establish aetiology of recurrent severe episodes in adults when considering referral for tonsillectomy.

Scarlet fever
This is also caused by GABHS, and there has been a stark increase in scarlet fever recently, with 12,906 cases in England during 2015/16 — the highest level since 1969, with 89% of cases reported in children under 10 years of age [5].

Diagnosis can usually be made by symptoms alone, which include [6]:

  • Sore throat.
  • Headache and fever.
  • Pinkish/red sandpapery rash appearing within a day or two, typically on the chest and stomach but then spreading to other parts of the body.

Symptoms usually clear up after a week, and the majority of cases will resolve without complication, as long as the recommended course of antibiotics is completed. Potential complications include ear infection, throat abscess and pneumonia [6].

Pharmacy testing
So, if we can convince the 3 million people with sore throat to visit a pharmacy instead of the GP, given the prevalence and possible complications of GABHS, the available methods of diagnosis and their accuracy, the prognosis for people with sore throat and the threat of antibiotic resistance. Is swabbing the best way forward for patients? For pharmacy?

* This is an estimate based on the GP visits for all causes of sore throat
Note: The figure for people visiting their GP for sore throat in England has recently been quoted as 1.2 million people per year. However, I could find no accompanying reference.


[1] Scottish Intercollegiate Guidelines Network. Management of sore throat and indications for tonsillectomy. April 2010.
[2] Worrall GJ. Acute sore throat. Can Fam Physician. 2007 Nov; 53(11): 1961–1962. 
[3] NICE CKS. Sore throat – acute. July, 2015.
[4] OSOM Strep A test.
[5] Public Health England. Increase in scarlet fever across England. March, 2016.
[6] Public Health England. Scarlet fever: guidance and data. July, 2014.

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