Infection Tests Flashcards
What is the first most important step when diagnosing an infection?
History
Symptoms suggestive of infection can be “system specific” or “non-specific”.
What are examples of system specific symptoms?
Cough (*respiratory) Neck stiffness (*CNS) Bony pain (*orthopaedic) Skin pain/redness (*SST) Dysuria (*urinary)
What are examples of non specific symptoms?
Fever “burning up”
Shaking episodes/chills
Sweating/night sweats
Feeling muddled/confused
When symptoms suggest a diagnosis of infection other information can help identify potential pathogens.
What questions should be asked?
- Travel?
- Inside Uk
- Outside Uk (detail) - Occupation?
- Farmer, fishmonger, vet, air steward etc - Animal contact?
- Which animal(s)/nature of contact - Hobbies/past times?
- Sexual history?
What is considered as a fever/high temperature?
Temperature >38.0 C
What are general symptoms of infection?
Fever “burning up”
Chills, sweats, night sweats
Rigors
Examination findings can also be “system specific” or “non-specific”.
What are examples of system specific examination findings?
Lung crackles (*respiratory) Meningism (*CNS) Bony tenderness (*orthopaedic) Skin erythema (*SST) Loin tenderness (*urinary)
What are examples of non specific examination findings?
Pyrexia
Witnessed rigor/chills
Sweating
Confusion
What components of the blood are tested in a full blood count?
- RBCs
- WBCs
- Platelets
- Hb
Normocytic vs normochromic anaemia?
Normocytic anaemia: when the RBCs are of normal size
Normochromic anaemia: when conc of Hb is normal but insufficient RBCs
Is a FBC useful in infection testing?
Not really:
Hb: Not much help diagnosing infection – but anaemia of chronic disease (normocytic, normochromic) can be caused by infection
WCC: can be raised in infection, but other conditions too (poor specificity). Severe sepsis can lower WCC
How does severe sepsis affect WCC?
Reduces it
In a bacterial infection, describe the:
a) WCC
b) lymphocytes
c) neutrophils
a) raised
b) normal or low
c) raised
In a viral infection, describe the:
a) WCC
b) lymphocytes
c) neutrophils
a) normal
b) raised
c) normal
COVID-19 is an exception. Describe the:
a) WCC
b) lymphocytes
c) neutrophils
a) decrease
b) decrease
c) increase
What are inflammatory markers?
Elements of the innate immune system:
a) C-reactive protein
b) Procalcitonin
What is procalcitonin?
Procalcitonin (PCT) is the precursor of the hormone calcitonin, which in normal metabolic conditions is mainly produced by the C‐cells of the thyroid medulla.
The blood of healthy individuals contains only very low levels of PCT.
Describe PCT levels during bacterial infection
The level of PCT increases in response to a pro-inflammatory stimulus, especially of bacterial origin.
The high PCT levels produced during infection are not followed by a parallel increase in calcitonin or serum calcium levels.
What does an increase in PCT suggest?
Can be serious bacterial infections
What is CRP?
C-reactive protein is a substance produced by the liver in response to inflammation.
What would cause an increase in CRP?
CRP levels can arise when you have a viral infection. But they don’t go as high as during a bacterial infection (PCT more useful for this)
What is lung consolidation?
Occurs when the air that usually fills the small airways in your lungs is replaced with something else. Depending on the cause, the air may be replaced with: a fluid, such as pus, blood, or water. a solid, such as stomach contents or cells.
Which organ is responsible for the major part of lactate metabolism?
Liver
What can high blood lactate levels indicate?
Inadequate oxygen delivery and tissue hypoxia results in increased lactate generation:
- Strenuous exercise
- Severe infection (sepsis)
- Shock
- Heart failure
- Respiratory failure
These conditions lower the flow of blood and oxygen throughout the body.
What is CURB-65?
Aa clinical prediction rule for predicting mortality in community-acquired pneumonia and infection of any site.
What does CURB-65 consist of?
Confusion Urea >7 mmol/L Respiratory Rate >30 BP S<90mmHg, D≤60mmHg Age > 65
If the answer is ‘yes’ to any of these, you give 1 point
More points = more severe
What is the most common bacterial cause of community acquired pneumonia?
Streptococcus pneumoniae
What are the 3 methods of microbiological diagnosis?
- Culture
- Direct detection
- Immunological tests
What is does ‘culture’ involve?
A microbiological culture is a method of multiplying microbial organisms by letting them reproduce in predetermined culture medium under controlled laboratory conditions.
What does isolating a viable pathogen by culture enable?
Identification - Immediate or by further testing
Typing - To establish organism relatedness
Sensitivity testing - To direct antimicrobial therapy
What are the negatives of culture?
Not applicable to non-cultivable micro-organisms
Needs to be done before antibiotics are started
How long should the blood culture be incubated for?
Incubate at 35-37 degrees for 5-7days
How is microbial growth detected in blood culture?
Automatic monitoring of CO2
What is the time to positivity (TTP) in blood culture for significant bacteraemia?
12-24 hours
N.B. may be shorter in overwhelming sepsis or longer with fastidious organisms
What is the Gram stain?
Chemical process that distinguishes between bacterial cell walls that retain crystal violet, and those that do not, when stained and washed with acetone.
What colour does Gram-positive bacteria stain?
Purple
What colour does Gram-negative bacteria stain?
Pink (or colour of counter stain)
What is sensitivity testing?
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
What is the ‘zone of inhibition’?
A circular area around the spot of the antibiotic in which the bacteria colonies do not grow –> due to presence of a drug that impedes their growth
Used to measure the susceptibility of the bacteria towards the antibiotic.
What are the uses of sensitivity testing?
To inform decisions on targeted antimicrobial therapy
- Initial treatment is with “empiric” therapy
- Subsequent treatment is “targeted”
Limitation of sensitivity testing?
The correlation between antimicrobial sensitivity and clinical response is not absolute
Uses of blood culture?
- 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
Limitations of blood culture?
- Cultivable organisms only
- Is usually slower than direct detection
What does direct detection involve?
Detection of whole organism (microscopy)
Detection of component of organism (antigen, nucleic acid)
What is an example of an antigen that can be directly detected?
Legionella –> Legionella antigen detection test
Sensitivity vs specificity?
Sensitivity: the ability of a test to correctly identify patients with a disease.
Specificity: the ability of a test to correctly identify people without the disease.
Limitations of direct detection?
Does not give any information on:
- Antimicrobial susceptibility
- Typing
Uses of direct detection?
- Is usually the fastest diagnostic method
- Allows the use of appropriate empiric antimicrobial therapy
What is involved in immunological tests?
Detection of immune response to infection:
- Antibody detection
- Other immunological tests e.g. IFN-γ release assays in tuberculosis
What is involved during antibody detection in immunological tests?
IgM detection
Seroconversion (change from negative to positive result from one test to a subsequent test)
Fourfold rise in titre: rise in concentration of antibody from one test to a subsequent test
Define seroconversion
A change from a seronegative to a seropositive condition
Limitations of antibody testing?
Is restricted to patients with a detectable antibody response
Is retrospective –> Often too late to inform antimicrobial therapy decisions
