Microbiology Nuts & Bolts
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Diagnosing Infection

It may sound obvious but in order to manage a patient with an infection safely and effectively you first have to work out what is wrong with them. This is done through the dynamic process of formulating a differential diagnosis. The process begins the moment the patient is referred (e.g. you are told the patient has a cough you narrow questioning to the respiratory system and a differential diagnosis that includes diseases like pneumonia, lung cancer, COPD etc). By taking a history, examination and requesting targeted investigations you narrow down the differential diagnosis until you get to a single diagnosis. 

A differential diagnosis is a list of potential diseases or infections that a patient might have. The simplest and most effective method of formulating a differentia diagnosis is:
  • Immediately life threatening conditions e.g. meningitis, encephalitis, necrotising fasciitis 
  • Common conditions e.g. UTI, pneumonia, cellulitis, heart failure
  • Uncommon conditions e.g. infective endocarditis

In the History the key infection related aspects to concentrate on are:
  • The patient's specific symptoms - e.g. cough indicating chest or upper respiratory tract or right upper quadrant pain indicating possible cholangitis
  • A chronological timeline of when and how symptoms developed - e.g. Chicken Pox followed by haemorrhagic skin lesions pointing towards invasive Group A Beta-haemolytic Streptococcus and necrotising fasciitis
  • Contact with people with infections OR symptoms - e.g. Tuberculosis contact
  • A list of recent travel (countries and regions) - e.g. Malaria endemic regions
  • The patient’s vaccination history - e.g. primary infant courses as well as travel related vaccines
  • The patient’s current and former occupations - e.g. healthcare staff and blood borne viruses or plumbers and exposure to Legionella pneumophila
  • The patient’s pastimes and hobbies - e.g. water sports and exposure to rats in leptospirosis
  • Any pets OR contact with animals - zoonotic infections e.g. Pasteurella multocida and cat bites, Chlamydia psittaci and parrots
  • A sexual history - e.g. sexually transmitted diseases and blood borne viruses
  • The patient’s ethnic origin - e.g. exposure to relatives with tropical infections
  • The patient’s country of birth - e.g. chronic tropical infections or exposure to relatives with tropical infections

In the Examination the key infection related aspects to concentrate on are:
  • General observations - e.g. inhalers, bedside medications, finger clubbing, splinter haemorrhages or presence of new onset confusion
  • Presence and pattern of fever - e.g hyperthermia (>38°C) OR hypothermia
  • (<36°C), Vivax and Ovale malaria fever every 48 hours, Malariae malaria fever every 72 hours, Falciparum malaria fever more frequent than 24 hours
  • Heart rate - e.g. tachycardia >90 bpm in sepsis, BEWARE Beta-blockers falsely decrease heart rate OR arrhythmias can falsely increase heart rate
  • Blood pressure - e.g. systolic pressure <90mmHg OR decreased >40mmHg from baseline, BEWARE “normotensive” patients may actually be “hypotensive” if they are normally “hypertensive”
  • Skin lesions - e.g. blanching and non-blanching rashes can occur in meningococcal sepsis, erythema marginatum occurs in rheumatic fever, BEWARE symptoms more severe than clinical features consider necrotising fasciitis
  • Upper respiratory tract - e.g. tonsillar pus and presence of quinsy or swelling and erythema of the tympanic membrane in otitis media
  • Chest - e.g. decreased expansion, dullness to percussion, bronchial breathing and increased vocal resonance (tactile vocal fremitus) in consolidation
  • Heart - e.g. new murmurs in infective endocarditis, BEWARE if severe valve damage, there may be no murmurs as no turbulence of blood flow
  • Abdomen - e.g. right upper quadrant pain in cholangitis or loin pain in pyelonephritis
  • Central nervous system - e.g. acute confusion in the elderly (also part of CURB-65 in CAP) or neck stiffness and meningism in meningitis

Diagnosing Infection: Non-Microbiological Investigations include:
Diagnosing Infection: Non-Microbiological Investigations
Diagnosing Infection: Non-Microbiological Investigations
Microbiology Nuts & Bolts on Amazon
Topics in Basic Concepts:
  • What is Infection? Infection vs. Colonisation vs. Contamination
  • Source of Infection: Endogenous vs. Exogenous
  • Bacteraemia vs. Septicaemia
  • Types of Infectious Microorganisms
  • The Anatomy of a Bacterium    
  • What is Normal Flora and why is it Important?
  • Circumstances Affecting Normal Flora
  • How Antibiotic Prescribing Influences Normal Flora and the Ward Environment
  • Bacterial Flora in a Normal Person in the Community
  • Bacterial Flora in a Normal Person in a Hospital or Long-term Care Facility
  • Significance of Bacteria in the Bloodstream (Bacteraemia)
  • Diagnosing Infection: History
  • Diagnosing Infection: Examination and Non-Microbiological Investigations
  • Immunodeficiency States

​All these topics are covered in the book...Ready to buy your copy? Click here to buy your copy of "Microbiology Nuts & Bolts" Its updated and amazingly only slightly larger as its got 1/3 more in it! (11cmx18cmx2.5cm).

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