Blind Treatment of Bacterial Infection

If a bacterial infection is suspected, it is often impracticable to wait for test results before starting treatment. Selecting the 'best guess' antibiotic should be guided by the following principles:^1,2

• 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 condition is bacterial or viral, but this is not always easy. Consider delayed antibiotics. Some viral conditions may need prophylaxis to prevent secondary bacterial overgrowth, e.g. acute necrotising ulcerative gingivitis secondary to herpes simplex infection.
• 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 cerebro-spinal fluid. Where clinically appropriate, consider FBC, ESR, C-reactive protein, urea and electrolytes, liver function tests, clotting, atypical serology, malaria film, serum for virology, chest X-ray, and arterial blood gas analysis. Stick test the urine.
• 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.
  • Excessive dose may result in toxicity, particularly for antibiotics with a narrow margin between the toxic and therapeutic dose (e.g. 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, though some patients have difficulty swallowing tablets and may need liquid or dispersible preparations. Serious infections may require intravenous administration. Avoid intramuscular antibiotics in children as these are likely to be painful.
• Duration depends on condition and severity. Chronic infections such as TB may require prolonged treatment. Upper respiratory infections usually require short courses. Follow local policy and national guidelines.³
• Consider any other factors relating to the patient likely to be relevant, e.g. ethnicity, history of allergy, whether immunocompromised, severity of condition, and whether taking other medication.
• If female:
  • Check whether pregnant, breast-feeding or taking oral contraceptive.
  • In pregnancy avoid tetracyclines, aminoglycosides, quinolones, high dose metronidazole.
  • Short-term use of trimethoprim (theoretical risk in first trimester in patients with poor diet, as folate antagonist) or nitrofurantoin (at term, theoretical risk of neonatal haemolysis) is unlikely to cause problems.
• Prescribing antibiotics after a telephone consultation should be the exception rather than the rule.
• 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 get gastro-intestinal side-effects with erythromycin.
• If blind treatment fails and test results are not available, check with a microbiologist.

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

• History:
  • A detailed history may reveal the source of infection.
  • Ask about respiratory, gastro-intestinal or genito-urinary symptoms.
  • Ask about recent travel or treatment or conditions which could compromise the immune system.
• Examination - check vital signs: temperature, pulse, BP, respiratory rate and capillary return, to assess the severity of illness and signs of septicaemia.
• Treatment:
  • After 'best guessing' the source of infection, follow local guidelines.
  • If none exists, use the guidance from the Health Protection Agency (HPA).³
  • Be ready to change treatment once drug sensitivities are known.
  • Treatment of most infections should not exceed 7 days.
  • In a hospital or intermediate care setting, intravenous antibiotic therapy is usually reviewed after 48 hours and changed to oral preparations when possible.
  • If in doubt, ask a microbiologist.

The table is a brief summary. Guidance changes from time to time depending on prevailing antibiotic sensitivities. Check the HPA for the most up to date guidance and details of the management of specific conditions.³

1. Wong SY, Lam MS; Pyrexia of unknown origin--approach to management.; Singapore Med J. 1995 Apr;36(2):204-8. [abstract]
2. 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. [abstract]
3. Management of Infection - Guidance for Primary Care; Management of Infection - Primary Care Guidance, Health Protection Agency (various dates); Guidelines for primary care (including diagnosis - quick reference guides)

• Community Management of Lower Respiratory Tract Infection in Adults, SIGN (2002)
• Bandolier; Reducing Antibiotic Prescribing 2000