Week 4 - Diagnosis and Pathogenesis of Infectious Diseases

History taking & physical assessment

history taking subjective data (symptoms)

physical assessment is objective data (signs)

Does the lab result mean the diagnosis itself?

a lab result supports diagnosis only when it matches the clinical picture and comes from a high-quality specimen

• body site and patient findings matter

Clinical Specimen: blood

blood is normally sterile

• it is commonly a bloodstream infection (bacteremia); sepsis evaluation

• nursing focus → aseptic collection; correct bottles for culture; timing

Clinical Specimen: Nursing

urinary tract infection (UTI)

• clean-catch midstream or ordered method; prompt transport (need to make sure urine is new, prevents microbes from being in there → clean-catch)

Clinical Specimen: cerebralspinal fluid (CSF)

meningitis (inflammation of the meninges), encephalitis (inflammation of the brain tissue)

• nursing focus: urgent transport (can lead to severe complications), follow strict protocols

• CSF is collected when meningitis, encephalitis, or meningoencephalitis is suspected

• preliminary results may need to be communicated rapidly

• follow facility proper for collection, labeling, and immediate transport

when doing a spinal tap, the patient should be lying on their side in the fetal position, the needle needs to be sterile, so you don’t contaminate the sample and introduce microbiota into the spine

Clinical Specimen: Sputum

lower respiration infection (e.g, pneumonia)

• nursing focus: sputum (not salvia); quality matters

  • saliva contains oral microorganisms

sputum samples need to be collected from lower respiratory secretions

• saliva has oral microbiota rather than lower respiratory ones, can make results inaccurate

• ask the patient to take deep breaths, then ask them to cough deeply, drinking water helps

Clinical Specimen: wound/tissue/fluids

abscess, surgical site infection

• nursing focus: collect from infected sit; avoid surface contamination

collect from the site where the pathogen is most likely to be found

• deep tissue aspirate or tissue is often more meaningful than superficial swab for deep infection

• if anaerobes are suspected, use appropriate anaerobic collection/transport

What does proper selection, proper transport, and proper collection do to specimen quality

proper selection → collect from the site where the suspected pathogen is most likely to be found

• ex) wound infection → take sample from wound

proper collection → reduce contamination from indigenous microbiota

proper transport → preserve organism viability and prevent overgrowth or degeneration

• leaving the specimen can cause bacteria to overgrow and not be accurate

Should you collect specimen before antimicrobials when possible?

yes you should

• the antimicrobials reduce organism recovery in culture

• do not delay urgent treatment when waiting would harm the patient

collect cultures before antimicrobials when feasible, but do not delay urgent treatment

What is a common error during the selection step?

wrong specimen type or site

• this matters because the sample may not represent the suspected infection site

What is common error during the collection step?

poor aseptic technique or inadequate site preparation

• need to clean the area first, make sure the needle is sterile

• this matters because contamination with indigenous microbiota

What is common error during the labeling step?

missing or incorrect patient ID, source, or collection time (can cause lab to reject the specimen)

• this matters because the specimen may be rejected or the results may be misattributed or misinterpreted

What is common error during the transport step?

delay, wrong temperature, or oxygen exposure

• this matters because overgrowth, organism death, or reduced recovery

What is common error during the request step?

missing test order or relevant clinical information

• this matters because appropriate testing or processing may be delayed or limited

Standard precautions during specimen handling

• hand hygiene

• treat blood, body fluids, secretions, excretions, nonintact skin, and mucous membranes as potentially infections

• Use PPE based on the anticipated exposure

• protect the patient, yourself, couries, and lab personnell

Nurse’s role in diagnostic process

• need to verify the order and correct specimen course

• use correct technique and reduce contamination (ex., aseptic technique for blood culture)

• label correctly and transport promptly

• recognize urgent or inconsistent results and communicate concerns

Why does aseptic technique matter for blood cultures?

the blood is normally sterile, so bacteria within the blood (bacteremia) may indicate infection

• also, sepsis is a concern because an infection in the blood is a systemic issue

• skin microbiota can contaminate blood cultures if technique is poor

a positive blood culture always warrants prompt clinical interpretation

do not dismiss concerning organisms from a normally sterile site

consider the organisms identity, number of positive bottles/sets, when the cultures were collected, and the patient’s condition

Urine culture - clean catch and colony count

clean catch is when you let the patient pee a little first to get rid of the bacteria, then catch the pee in mid-stream

• UTI diagnosis depends on the symptoms, specimen quality, and microbiology

• complete urine culture may include colony count, identification, and susceptibility testing

• delayed transport without appropriate refrigeration or preservation can increase the colony count

Urine results: some follow-up questions

are there compatible symptoms? this helps distinguish asymptomatic bacteria from symptomatic UTI (ex., dysuria, frequency, fever, and flank pain)

was the specimen collected properly? poor technique increases contamination risk (not clean-catch and mixed flora)

what organism and count? helps decide clinical significance (ex., E coli at high colony count)

was transport appropriate? delay can change colony count (room-temperature urine left for hours)

From specimen to result

1. obtain a pure culture of the suspected pathogen

2. perform tests necessary to identify suspected pathogen

3. perform antimicrobial susceptibility testing

4. report findings to clinician

Culture-based testing

grows the organism before identification

• however, some tests can detect or identify microbes directly from the specimen without first growing them in culture

Identification of a specimen

seeing which organism is present

• uses growth characteristics, biochemical tests, mass spectrometry, or PCR (polymerase chain reaction)

• you interpret the results with the infection site, patient factors, and clinical findings

Susceptibility of a specimen

asking what antimicrobials are likely to inhibit it

Preliminary findings

early finding that may guide urgent cart while workup continues

Final report

completed identification and other finalized test results when testing is complete

Pathogenicity

ability of a microbe to create disease

• organism-host relationship

Pathogenesis

steps/mechanisms involved in disease development

• describes what happens from exposure to tissue damage

Infection

the pathogen becomes established in the host (colonization)

• may be asymptomatic and does not always lead to disease

• ex) latent TB

Infectious disease

infection results in disease

• produces tissue damage and/or clinical manifestations

• requires interpretation of patient findings or lab testing

• ex) active TB

Why does exposure not always lead to disease?

• the microbe might land at a site where it cannot multiply

• the microbe may fail to attach to required receptors

• antimicrobial factors, indigenous microbiota, and phagocytes may inhibit the microbe

• the host may already be immune

• nutrition, immune status, and overall health influence

Chain of infection

1. source of infection (the pathogen)

2. Reservoir (where the pathogen lives)

3. Portal of exit (ex., when the host sneezes)

4. Mode of transmission (ex., droplets)

5. Portal of entry

6. Susceptible host

Course of an infectious disease

1. Exposure to pathogen

2. Incubation period → period between when the patient is exposed to when they first show signs and symptoms

3. Prodromal period → when. the patient feels like they are coming down with something (bacteria is multiplying)

4. Period of illness → active infection

5. Can go 1 of 3 ways → convalescence (full recovery, signs and symptoms subside), disability, or death (infection becomes systemic and becomes deadly)

Localized infections

limited to one body area (ex., abscess, boil, local wound infection)

• ringworm (tinea) is an example

localized infections can turn into systemic infections by spread/invasion and host response

Systemic or generalized infection

affects the body beyond one site (blood infections)

• ex) disseminated infection or systemic viral illness

some cues of a systemic infection is fever, chills, tachycardia, hypotension, altered mental status)

Acute infection

has a rapid onset, severe symptoms, and is short lived

• ex) influenza, cold

Chronic infection

slow onset and longer duration with more mild/no symptoms

• TB; syphiliis

Signs vs symptoms

signs → the things you can observe and measure (fever, abnormal pulse, abnormal lab results)

• important for cue recognition and prioritization

symptoms → the feelings the patient tells you (headache, pain, nausea, itching, chills, dizziness)

• important for nursing assessment and history

Latency vs recurrence

latency → the pathogen remains in the host without any signs or symptoms

recurrence (reactivation) → the pathogen becomes active again, causing signs and symptoms to recur (ex., cold sore → recurrent herpes infection)

key idea: absence of symptoms does not always mean the pathogen has been eliminated

Primary infection

the initial infection that starts the problem

• may damage barriers or alter microbiota

Secondary infection

new infection that occurs during or after the primary infection, often when the defenses are impaired

ex) bacterial pneumonia after flu

Common pathogenesis network

1. Entry of the pathogen into the body → by respiratory tract, gastrointestinal tract, genitourinary tract, breaks in skin, mucous membrane, placenta, and parenteral routes

2. Attachment of the pathogen to some tissues within the body

3. multiplication of the pathogen

4. invasion or spread of the pathogen

5. evasion of the host defenses

6. damage to host tissues

portal of exist → respiratory secretions, feces, urine, blood, drainage, genital secretions, and skin shedding

Virulence

degree of pathogenicity

• the relative ability to cause disease or damage

• these factors help pathogens attach, evade host defenses, spread, or cause damage

• ex) adhesions, capsules, exoenzymes, toxins, antigens, variation, intracellular survival

Virulence Factors by function

attach; ex) adhesions, fimbriae/pili, clinical meaning = helps pathogen stay at correct site

evade defenses; ex) capsule, antigenic variation, intracellular survival; clinical meaning = reduces host clearance

spread/invade; hyaluronidase, collagenase, kinases; clinical manifestation = supports tissue penetration or spread

damage host; ex) exotoxins, endotoxins, hemolysis, leukocidins; clinical manifestations = causes signs, symptoms, or systemic inflammation

Attachment of a virus

when the ligand meets the receptor

• attachment is specific, viruses require specific receptors to attach and bind to host

• without attachment, the pathogen cannot colonize or cause disease

• receptor specificity helps explain why pathogens infect particular tissues or hosts

Pili/fimbriae

supports attachment of the bacteria

• supports virulence

Capsules

helps bacteria evade phagocytosis (immune cells enter by phagocytosis)

• helps with virulence b/c it helps bacteria survive host defenses

makes bacteria harder for phagocytosis to engulf

clinical connection: losing the capsule can reduce resistance to phagocytosis

Flagella

helps/supports movement of bacteria

• promotes virulence

Enzymes, toxins, and LPS

can promote spread of bacteria or host damage

• helps with virulence

Exoenzymes

they support and survival

hyaluronidase → breaks down hyaluronic acid, this matters because it helps microbes through connective tissue

collagenase → breaks down collagen, this matters because tissue invasion and damage

kinases → breaks down clots/fibrin, this matters because it may help spread through tissue

coagulase → promotes clot formation, this matters because it may wall bacteria from defenses

Exotoxin

source: protein secreted by some bacteria

typical action: often targets specific cells or body functions

clinical effects: depends on the toxin, such as neurologic, GI, or cell injury

ex) neurotoxins, enterotoxins, and leukocidins

Endotoxin

source: lipopolysaccharides (LPS) in the outer membrane of gram (-) bacteria

typical actions: triggers strong inflammatory responses

clinical effects: fever, inflammation, hypotension, and possible shock

Toxin Types

neurotoxins → affect NS function

Enterotoxins → affect the GI tract and cause vomiting/diarrhea

Leukocidins → damage the white blood cells

Pyrogenic → can trigger fever (only some cells)

Antigenic variation

changes the surface antigens to avoid recognition

Molecular mimicry

resembles the host molecules, reducing immune detection or triggering autoimmunity

Intracellular survival

hides inside host cells where defenses or drugs may be less effective

Localized cues

redness, warmth, swelling, pain, drainage

Systemic cues

fever, chills, tachycardia, hypotension, altered mental status

Does pathogenesis explain how infection and the host response produce local and system clinical findings?

yes

note: not every localized infection progresses systemically