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Blind treatment of bacterial infection

Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Traveller's diarrhoea article more useful, or one of our other health articles.

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Bacterial infection treatment

If a bacterial infection is suspected, it is often impracticable to wait for test results before starting treatment. Selecting the most appropriate antibiotic should be guided by the following principles:1

  • Use antibiotics responsibly, considering issues such as safety, resistance and cost.

  • Check that an antibiotic is really needed - history and examination may yield clues as to whether a condition is bacterial or viral; however, this is not always easy. Consider delayed antibiotics. Some viral conditions may need prophylaxis to prevent secondary bacterial overgrowth - eg, acute necrotising ulcerative gingivitis secondary to herpes simplex infection.

  • C-reactive protein (CRP) blood test:

    • The use of point of care rapid CRP testing may reduce the prescription of antibiotics. There is evidence of an overall reduction in the use of antibiotics when using CRP tests to guide whether antibiotic treatment is required.2

    • However, the CRP result is nonspecific and should be considered in the context of the clinical presentation. The usefulness of the CRP level can be affected by various factors, including the age of the patient, site of infection and the timing of the test. CRP values are often slightly raised in the indeterminate range and therefore may be of limited value.

  • Blind prescribing does not obviate the need to take samples for culture and sensitivity, before starting treatment, whenever appropriate. Depending on the clinical picture, this may include skin or wound swabs, high vaginal swabs, endocervical swabs, urine, faeces, sputum, blood, aspirate. In the hospital environment, consider cerebrospinal fluid.

  • Where clinically appropriate, consider FBC, ESR, CRP, U&Es, LFTs, clotting, atypical serology, malaria film, serum for virology, CXR and arterial blood gas analysis. Perform urinalysis.

  • Blind antibiotic prescribing for pyrexia of unknown origin (PUO) in a relatively well and stable patient is rarely helpful.

  • Calculating dosage is not an exact science but consider factors affecting absorption or bioavailability, such as age, weight, hepatic function, renal function, severity of infection and other medication:

    • Underdosing may result in significant failure of treatment and bacterial resistance in serious infection.

    • An excessive dose may result in toxicity, particularly for antibiotics with a narrow margin between the toxic and therapeutic dose (eg, an aminoglycoside).

    • Consider drug plasma monitoring, although this is difficult in primary care and may be more appropriate in an intermediate care setting.

  • Route of administration - most patients in primary care will cope with oral antibiotics, although some patients have difficulty swallowing tablets and may need liquid or dispersible preparations. Serious infections may require intravenous (IV) administration. Avoid intramuscular (IM) antibiotics in children, as these are likely to be painful.

  • Duration depends on condition and severity. Chronic infections such as tuberculosis may require prolonged treatment.

  • Follow local policy and national guidelines.3

  • Consider any other factors relating to the patient which are likely to be relevant - eg, ethnicity, history of allergy, whether immunocompromised, severity of condition and whether taking other medication.

  • If female:

    • Check whether pregnant, breast-feeding or taking an oral contraceptive.

    • In pregnancy avoid tetracyclines, aminoglycosides, quinolones, high-dose metronidazole.

    • Short-term use of trimethoprim (there is a theoretical risk in the first trimester in patients with a poor diet, as it is a folate antagonist) or of nitrofurantoin (at term, there is a theoretical risk of neonatal haemolysis) is unlikely to cause problems.

  • Prescribing antibiotics after a telephone consultation should be the exception rather than the rule.An exception for this is for uncomplicted urinary tract infections in women witih typical symptoms, in which case a urine dipstick and examination is not needed.4

  • Choose simple generics first-line unless there is a very good case for using newer more expensive antibiotics.

  • Avoid widespread use of topical antibiotics, especially those readily used in oral forms, as this may spread resistance.

  • Clarithromycin is an acceptable alternative in patients who have gastrointestinal side-effects with erythromycin.

  • If blind treatment fails and test results are not available, check with a microbiologist.

Which anti-infective?3

Choosing the right drug in the absence of sensitivity results is an inexact science at the best of times but should be guided by the following principles:


  • A detailed history may reveal the source of infection.

  • Ask about respiratory, gastrointestinal or genitourinary symptoms.

  • Ask about recent travel or treatment or conditions which could compromise the immune system.


Check vital signs: temperature, pulse, blood pressure, respiratory rate and capillary return, to assess the severity of illness and signs of septicaemia.


  • After 'best guessing' the source of infection, follow local guidelines.

  • Be ready to change treatment once drug sensitivities are known.

  • Treatment of most infections should not exceed seven days.

  • In a hospital or intermediate care setting, IV antibiotic therapy is usually reviewed after 48 hours and changed to oral preparations when possible.

  • If in doubt, ask a microbiologist.

Continue reading below

Management of infection guidance for primary care from PHE - now UK Health Security Agency3

Unless otherwise specified, the antibiotic doses in the following table are for adults. Always check a drug formulary such as the British National Formulary for Children when prescribing for children.5

Blind Treatment of Infection




Most sore throats are viral, but if bacterial tonsillitis is suspected:

Phenoxymethylpenicillin 500 mg QDS or 1 g BD for 10 days.

If allergic to penicillin, clarithromycin 250 mg-500 mg BD for 10 days.

Otitis media in childhood7

Many are viral - 80% resolve without antibiotics. If clinically appropriate:

Amoxicillin first-line - 40 mg/kg/day in three divided doses.
Maximum 1 g TDS for five days.

If allergic to penicillin: Clarithromycin or erythromycin first-line.


Avoid antibiotics unless severe, or where symptoms are lasting more than 10 days. 80% resolve within 14 days without antibiotics.

First-line - phenoxymethylpenicillin 500 mg QDS for five days.

If allergic to penicillin, doxycycline 200 mg on the first day then 100 mg once daily for a further four days or clarithromycin 500 mg BD for five days.

If pregnant and allergic to penicillin, erythromycin 250-500 mg QDS for five days.

For persistent symptoms - co-amoxiclav 625 mg TDS for seven days - seek microbiology advice if allergic to penicillin.

Acute bronchitis/lower respiratory tract infection9

Only marginal benefits in otherwise healthy adults. Patient leaflets can reduce antibiotic use:

Doxycycline 200 mg stat then 100 mg OD for five days.

If the person is pregnant, amoxicillin 500 mg TDS for five days.

Alternatives include erythromycin or clarithromycin.

Acute exacerbation of chronic obstructive pulmonary disease10

Use antibiotics if there is increased dyspnoea


purulent sputum


increased sputum volume:

First-line - amoxicillin 500 mg TDS or doxycycline 200 mg stat then 100 mg OD or clarithromycin 500mg BD, for 5 days.

If resistance - co-amoxiclav 625 mg TDS for 5 days.

Community-acquired pneumonia9

Use CRB-65 score to guide appropriate management. See the separate Pneumonia article:

First-line: amoxicillin 500 mg-1 g TDS or clarithromycin 500 mg BD or doxycycline 200 mg stat the 100 mg OD for up to 10 days.

Consider risk factors for Staphylococcus aureus and Legionella spp.

Assess need for dual therapy for atypical organisms: amoxicillin and clarithromycin, or doxycycline alone.


Admit to hospital immediately:

Intramuscular or intravenous benzylpenicillin or cefotaxime prior to admission, unless there is history of anaphylaxis, (a history of a rash with penicillin is not a contra-indication).

Benzylpenicillin: adults and children 10 years and over: 1200 mg. Children 1-9 years: 600 mg. Children under 1 year: 300 mg.

Cefotaxime: age 12+ years: 1 gram. Age <12 years: 50 mg/kg.

Uncomplicated urinary tract infection (UTI) - ie no fever or flank pain12 13

Amoxicillin resistance is common; therefore, ONLY use it if culture confirms susceptibility.

In the elderly (>65 years), do not treat asymptomatic bacteriuria; it occurs in 25% of women and 10% of men and is not associated with increased morbidity.

In the presence of a catheter, antibiotics will not eradicate bacteriuria; only treat if systemically unwell or if pyelonephritis is likely.

Do MSU on all treatment failures - extended-spectrum beta-lactamase enzyme-producing organisms increasing multiple resistance but still sensitive to nitrofurantoin.

Uncomplicated UTI (no fever or flank pain):

Use nitrofurantoin or trimethoprim first-line

Trimethoprim 200 mg BD, nitrofurantoin 50-100 mg QDS or 100 mg modified-release BD for three days if the eGFR is at least 45,

Second-line choices include pivmecillinam 200 mg TDS (400 mg TDS if resistance risk) or fosfomycin 3 g as a single dose.

Treat for three days in women and for seven days in men.

UTI in pregnancy:

Send MSU for culture and start antibiotics.

Short-term use of nitrofurantoin in pregnancy is unlikely to cause problems to the fetus.

First-line - nitrofurantoin 100 mg MR BD (avoid at term).

Second line - amoxicillin 500 mg TDS (if organism susceptible); treat for seven days or cefalexin 500 mg BD for seven days.

Children (see also separate Urinary Tract Infection in Children article):

Lower UTI: trimethoprim, second-line - cefalexin, trimethoprim or amoxicillin.

Upper UTI: cefalexin or co-amoxiclav.

Recurrent UTI in non-pregnant women (three or more UTIs/year):

See also the separate Recurrent Urinary Tract Infection article.

Consider prescribing vaginal oestrogen in postmenopausal women.

For prophylaxis use nitrofurantoin 50-100 mg or trimethoprim 100 mg OD at night (review at six months).

Skin/soft tissue infections14 15 16


Reserve topical antibiotics for very localised lesions to reduce risk resistance.

Consider hydrogen peroxide 1% cream applied 2-3 times daily for five days for people who are not systemically unwell or at a high risk of complications.

If meticillin-resistant S. aureus (MRSA) is suspected, consult microbiology.

First-line oral medication - flucloxacillin 500 mg or, if penicillin allergy, clarithromycin 250-500 mg BD; treat for seven days.

Topical - use fusidic acid TDS for five days.


Using antibiotics, or adding them to steroids, in eczema does not improve healing unless there are visible signs of infection.


Flucloxacillin 500 mg QDS for even days. Use clarithromycin or doxycycline if there is allergy to penicillin.

If febrile and ill, admit for IV treatment.

If penicillin-allergic and on statins use doxycycline 200 mg stat then 100 mg OD for seven days (a pragmatic alternative for the statin/macrolide interaction is to omit the statin for the duration of the antibiotic use).

If unresolving: clindamycin 300-450 mg QDS seven days.

In facial cellulitis, use co-amoxiclav 625 mg TDS for seven days.

Initially treat for seven days but continue for a further seven days if there is slow response.

Leg ulcers:

Bacteria will always be present. Antibiotics do not improve healing unless there is active infection. Culture swabs and review antibiotics after culture results.

Active infection indicated if there is cellulitis, increased pain, pyrexia, purulent exudate or odour.

If there is active infection, use flucloxacillin or clarithromycin as for cellulitis.

Human and animal bites:

Thorough irrigation is important.

Assess tetanus, rabies, HIV and hepatitis B/C risk.

Consider whether the bite and/or the person are high-risk. High-risk bites include those in the hands, feet, genitals, skin overlying cartilaginous structures or an area of poor circulation. High-risk people include those who have a comorbidity that increases the risk of serious wound infection, for example diabetes, immunosuppression, asplenia or decompensated liver disease.

Do not offer antibiotics for bites that have not broken the skin.

Consider antibiotics for bites which have broken the skin but not drawn blood if the bite is from a human and the person/bite is high-risk, or from a cat if the wound could be deep.

Offer antibiotics for human and cat bites that have broken the skin and drawn blood. Only offer antibiotics for dog bites in this situation if the bite has caused considerable deep tissue damage or is visibly contaminated (eg, with dirt or a tooth), or the person or bite is high-risk,

First-line is co-amoxiclav 375-625 mg TDS for seven days.

Second-line (for patients with penicillin allergy, or if co-amoxiclav is otherwise unsuitable) Metronidazole 400 mg TDS PLUS doxycycline 100 mg BD

Seek specialist advice if antibiotics are indicated and the person is pregnant and allergic to penicillin.

NB: doses are for adults unless otherwise stated - for further details see the British National Formulary.

The table is a brief summary. Guidance changes from time to time depending on prevailing antibiotic sensitivities and it may also be appropriate to consult local guidance, which may take into account local sensitivity issues.

Further reading and references

  1. Cunha BA; Fever of unknown origin: focused diagnostic approach based on clinical clues from the history, physical examination, and laboratory tests. Infect Dis Clin North Am. 2007 Dec;21(4):1137-87, xi.
  2. Smedemark SA, Aabenhus R, Llor C, et al; Biomarkers as point-of-care tests to guide prescription of antibiotics in people with acute respiratory infections in primary care. Cochrane Database Syst Rev. 2022 Oct 17;10(10):CD010130. doi: 10.1002/14651858.CD010130.pub3.
  3. Managing common infections: guidance for primary care; Public Health England, August 2020 - last updated June 2021
  4. Diagnosis of Urinary Tract Infections. Quick reference tool for primary care for consultation and local adaptation; Public Health England 2020
  5. British National Formulary (BNF); NICE Evidence Services (UK access only)
  6. Sore throat - acute; NICE CKS, May 2024 (UK access only)
  7. Otitis media - acute; NICE CKS, January 2021 (UK access only)
  8. Sinusitis; NICE CKS, March 2021 (UK access only)
  9. Chest infections - adult; NICE CKS, June 2021 (UK access only)
  10. Chronic obstructive pulmonary disease; NICE CKS, November 2021 (UK access only)
  11. Meningitis - bacterial meningitis and meningococcal disease; NICE CKS, March 2024 (UK access only)
  12. Urinary tract infection (lower) - women; NICE CKS, March 2023 (UK access only)
  13. Urinary tract infection - children; NICE CKS, July 2022 (UK access only)
  14. Impetigo; NICE CKS, February 2020 (UK access only)
  15. Human and animal bites: antimicrobial prescribing; NICE Guidance (November 2020)
  16. Cellulitis - acute; NICE CKS, March 2024 (UK access only)

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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