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Y2 Clinical Pathology > Infection Tests > Flashcards

Infection Tests Flashcards

1
Q

Why are infection tests performed?

What happens if they are done well / poorly?

A

they are vital to the optimal treatment and prevention of infection

if they are done well, they can save lives, time and money

if they are done poorly, they can result in loss of life

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2
Q

What types of symptoms mentioned in a history are key to making a diagnosis of an infection?

A

System specific symptoms:

  • cough - respiratory
  • neck stiffness - CNS
  • bone pain - orthopaedic
  • skin pain / redness - skin/soft tissue
  • dysuria - urinary

Non specific symptoms:

  • help to support a diagnosis of infection
  • fever
  • shaking episodes / chills
  • sweating / night sweats
  • feeling muddled / confused
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3
Q

When symptoms suggest a diagnosis of infection, what other information can help identify potential pathogens?

A

Travel:

  • inside UK?
  • outside UK? Need details

Animal contact:

  • which animal (s) / nature of contact

Occupation:

  • e.g. farmer, fishmonger, vet, air steward, etc.

Hobbies / past times

Sexual history

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4
Q

What is fever a sign of?

What temperature is defined as having a fever?

A

Fever is a sign of inflammation and can be a symptom / sign of infection

Temperature > 38.0 o C

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5
Q

What are the symptoms of fever?

A
  • Fever - “burning up”
  • chills, sweats, night sweats
  • rigors
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6
Q

What is the purpose of examination in diagnosing infection?

What are examples of the 2 types of examination findings?

A

Examination can support diagnosis based on history

System specific findings:

  • lung crackles - respiratory
  • meningism - CNS
  • bone tenderness - orthopaedic
  • skin erythema - skin/soft tissue
  • loin tenderness - urinary

Non-specific findings:

  • pyrexia
  • witnessed rigor / chills
  • sweating
  • confusion
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7
Q

Why might you look at haemoglobin on a full blood count in someone with a suspected infection?

A

Anaemia of chronic disease (normocytic, normochromic) can be caused by infection

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8
Q

Why might white blood cell count (WCC) be a useful indication of infection?

A

WCC can be raised in infection, but in other conditions too so has poor specificity

severe sepsis can lower WCC

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9
Q

How will the white cells vary in bacterial and viral infections?

A
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10
Q

What are the 2 inflammatory markers and how does their presence affect diagnosis?

A

Inflammatory markers are elements of the innate immune system

C - reactive protein and procalcitonin

Raised markers support diagnosis of infection and negative markers make infection less likely

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11
Q

What are normal levels of the inflammatory markers?f

A

C-reactive protein:

  • < 5 mg/L

Procalcitonin:

  • < 0.5 micro gram / L
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12
Q

What types of scans are used to support diagnosis of infection?

A
  • X-rays
  • computed tomography (CT)
  • CT combined with PET
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13
Q

What tests can be used to identify severe infection?

A

Blood lactate and blood gases can help to identify severe sepsis and respiratory failure

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14
Q

What are the 3 methods of microbiological diagnosis?

A
  • Culture
  • direct detection
  • immunological tests
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15
Q

What does the isolation of viable pathogen through culture enable?

A

Identification:

  • immediately or by further testing

Typing:

  • To identify organism relatedness

Sensitivity testing:

  • to direct antimicrobial therapy
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16
Q

What are the 2 things that must be considered before performing culture?

A
  1. It is not applicable to non-cultivable micro-organisms
  2. It needs to be done before antibiotics are started
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17
Q

What are the 5 stages involved in blood culture sampling?

A
  1. Patient sampling
  2. Sample handling
  3. Specimen transport
  4. Incubation
  5. Growth detection
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18
Q

How can contamination be avoided during patient sampling?

How can sensitivity be maximised?

A

Contamination is minimised by aseptic technique

Sensitivity can be maximised by sampling the correct volume

19
Q

What should be paid close attention to during sample handling?

A

Follow local instructions for safety, labelling and number of samples / bottles required

20
Q

How long does the incubation phase last for?

How can microbial growth be detected?

A

Incubate at 35 - 37oC for 5 - 7 days

microbial growth is usually detected by constant automatic monitoring of CO2

if there is no growth, the specimen is negative and is discarded

21
Q

What measurement is used in growth detection?

How long does this take?

A

Time to positivity (TTP) is usually 12 - 24 hours in significant bacteraemia

it may be shorter in sepsis

it may be longer in fastidious organisms

22
Q

What are the final 5 stages in blood culture sampling after growth detection?

A
  1. Preliminary results
  2. Incubation
  3. Culture results
  4. Definitive results
  5. Reporting
23
Q

How are the preliminary results obtained in blood culture sampling?

Why is this stage important?

A

A Gram film of the blood culture medium is examined and results are communicated immediately to the clinician to guide antibiotic therapy

24
Q

What is involved in the 2nd incubation phase in blood culture sampling?

A

A small amount of the medium is incubated on a range of culture media

preliminary susceptibility testing may be carried out

25
Q

What is involved in the culture results and definitive results stages of blood culture sampling?

A

Culture results:

  • preliminary susceptibility results are communicated to the clinician

Definitive results:

  • further overnight incubation is often required for definitive identification of organisms by biochemical testing, and additional sensitivity testing
26
Q

What is meant by “Gram stain”?

What is the difference in colour between Gram-positive and Gram-negative?

A

A chemical process that distinguishes between bacterial cell walls that retain crystal violet, and those that do not, when stained and washed with acetone

Gram-positive:

  • purple

Gram-negative:

  • pink (or colour of counter-stain)
27
Q

What is required for sensitivity testing?

A

Sensitivity testing requires viable micro-organisms

usually bacteria or fungi

28
Q

What is the basic principle behind sensitivity testing?

A
  • Culture of micro-organism in the presence of antimicrobial agent
  • work out if the concentration of antimicrobial that will be available in the body is high enough to kill the micro-organism
  • use solid or liquid media
29
Q

What are the uses of sensitivity testing?

What are the 2 types of treatment that can be used as a result, and what do they require?

A

It informs decisions on targeted antimicrobial therapy

Initial treatment is with empiric therapy

Subsequent therapy is targeted

This requires isolation of the micro-organism and antimicrobial sensitivity testing

30
Q

What is the main limitation of sensitivity testing?

A

The correlation between antimicrobial sensitivity and clinical response is not absolute

31
Q

What are the 3 main uses of culture?

A
  • Establishes the presence of a micro-organism at a particular site
  • allows the use of empiric and targeted antimicrobial therapy
  • provides epidemiological and typing information
32
Q

What are the limitations of culture?

A
  • It is only suitable for cultivable organisms
  • it is usually slower than direct detection
33
Q

What are the principles of direct detection?

A

Detection of whole organism:

  • through microscopy

Detection of component of organism:

  • antigen
  • nucleic acid (DNA or RNA)
34
Q

What is the main antigen detection test?

What are the pros and cons of this test?

A

Legionella antigen detection test

it can be used as a point of care test as it gives rapid results at the bedside

it requires training and quality control can be a problem

35
Q

In what types of infections is a detection of nucleic acid test performed?

A

Viruses:

  • influenza

Bacteria:

  • Streptococcus pneumoniae
  • through 16S PCR

Fungi:

  • Candida spp.
  • Aspergillus spp.
36
Q

What are the 3 main uses of direct detection?

A
  • It establishes the presence of a micro-organism at a particular site
  • it can be used against cultivable and non-cultivable organisms
  • it allows the use of appropriate empirical antimicrobial therapy
  • usually the fastest diagnostic method
37
Q

What is the drawback of direct detection?

A

It doesn’t give any information on antimicrobial sensitivity or typing

38
Q

What is involved in an immunological test?

What are the 3 main types?

A

It involves the detection of immune response to infection

This is usually through antibody detection:

  1. IgM detection
  2. Seroconversion
  3. Fourfold rise in titre
39
Q

How does seroconversion work as a method of antibody detection?

A

Change from negative to positive result from one test to a subsequent test

40
Q

What is meant by “fourfold rise in titre” as a method of antibody detection?

A

The rise in concentration of antibody from one test to a subsequent test

“titre” is 1 / greatest dilution at which antibody is detected

i.e. if antibody is just detectable at a serum dilution of 1/64, then titre is 64

a fourfold rise in titre would be 2 –> 32 or 4 –> 64

41
Q

Other than antibody detection, what other immunological test is used?

A

IFN-y release assays in tuberculosis

42
Q

What are the uses of antibody testing?

A

It confirms exposure to a specific micro-organism

It can be used against cultivable and non-cultivable organisms

43
Q

What are the limitations of antibody testing?

A

It is restricted to patients with a detectable antibody response

It is retrospective - it is often too late to inform antimicrobial therapy decisions

Y2 Clinical Pathology (76 decks)

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