Bacterial disease testing Flashcards
Why are cultures performed?
- confirm a dx
- exclude a dx
- screening
- monitor the course of a disease
- monitor response to therapy
- stage the severity of the disease
- provide a prognosis
How can results of a culture be altered?
- Collection methods: swab, aspirate, expectorant
- physiologic variables
transient bacteremia - meds: especially abx
General culture interpretation guidelines
- ## as a general rule: infection considered if >10^5 organisms found
How long does it take cultures to grow?
1 full day to grow the organism and then part or all of 1 day to ID it. It may take an add. day to isolate it b/f ID if there is a mix of organisms.
- a preliminary report for most cultures may be issued in 24 hours
What is important to ID from the infectious organisms?
- normal flora from the infection
- normal flora and pathogens vary dependent on anatomic location
What is normal flora on the skin?
staph epiermidis, S. aureus, micrococcus, few gram - bacilli moist skin, corynebacterium, propionibacterium acnes
Pathogenic organisms on the skin?
strep. pyogenes
pseudomonas
proteus
Pathogens in the mouth?
strep pneumoniae, strep pyogenes, Neisseria meningitidis, H. influenza, N. gonorrheae
Potential pathogens and pathogens of oropharynx?
potentials: mycoplasma, bordatella pertussis, many others
pathogens: staph aureus, pseudomonas
normal flora in conjunctiva?
corynebacterium, Neisseria, moraxellae, staph, strep, occasional Haemophilus and parainfluenza
Pathogens of the conjunctiva?
pneumococcus, pseudomonas, strep
pathogens of the GI tract?
C. diff, salmonella, toxic strains of E. coli, Helicobactor pylori (in duodenum)
Pathogens of anterior urthera?
chlamydia, gonorrhea, syphilis
What does the normal flora of the vagina depend on?
- varies with hormonal state
Pathogens of the vagina?
candida, trichomonas
What are important steps to collecting a specimen and achieving the highest diagnostic yield?
- obtain specimen before abx
- use strict aseptic technique
- minimize contamination by skin and mucous membranes (be careful with swabbing)
- collect an adequate volume and send tissue or fluid rather then a swab when possible (culture when possible)
- label appropriately
- fill out requisition slips completely and precisely
- call microbio dept if you have any ???s
within how many hours of collection should samples arrive in the lab?
within 1-2 hours of collection, but if delay is unavoidable most specimens (except blood, CSF
Culture turnaround times?
blood cultures: 48-96 hours
most routine cultures (urine, throat, sputum): 24-48 hrs unless looking for unusual bacteria
- anaerobes: can take 48-72 hours
- almost always get gram stain with culture in most settings
appropriate culture media?
- routine: blood agar
- chocolate agar -> gram negatives (GC & Haemophilus)
- anaerobic blood agar: needs to be fresh
- sabourad agar: fungi
- GPS -> gram + selective media -> allows isolation of strep and staph and inhibits most gram - rods
- MacConkey or eosin methylene blue agars: anaerobic gram - rods
- Thioglycollate media: liquid media, enrichment broth used as supplement to plated media
Process of wound culture?
- usually pus present
- round up some pus on an applicator
- culture of specimens from the skin edge is less accurate than culturing the suppurative material
- if anaerobic organism suspected -> get anaerobic culturette from lab
Culture of abscess/boil/furuncle
- incised and any fluid or material swabbed with culture swab and sent to lab
- important to get pus/exudate from deep in the wound to avoid surface contamination
Culture of the eye-> conjunctiva
- gently swab to collect drainage
- place in approp. container and send to lab at 25 degrees C
How to obtain throat cultures? when are these done?
- done to rule out strep pharyngitis (GABS)
- sore throat, inflamed tonsils
- inform lab if trying to ID something else such as N. gonorrheae (this reqrs Thayer-Martin agar)
- swab posterior pharynx and tonsils
- avoid touching any other part of mouth
- send to lab at 25 degrees C
- often indicated if the in office rapid strep screen is negative
Sputum culture indications
- HAP
- hosp patients with pneumonia (CAP) and any of the following criteria:
- ICU admission
- failure of output abx therapy
- cavitary lesion
- active alcohol use (aspiration pneumonia)
- severe obstructive of structural lung disease
- urine antigen test for pneumococcus
- urine antigen test for Legionella
- pleural effusion
Sputum cultures are not indicated when:
- for most outpatient CAPs
- management of bronchitis
- initial management of acute exacerbations of COPD
How to obtain a sputum collection for optimal yield?
- obtain prior to abx tx
- have the pt rinse their mouth prior to expectoration
- no food 1-2 hours prior to expectoration
- inoculation of culture media immediately after specimen is obtained or immediately after prompt transport to microbio lab
- bronchoscopy: sampling of lower airway is another potential dx method in patients w/ suspected pneumonia
What may make a sputum culture inaccurate? When would you suspect a pathogen? When is the best time to get a sputum culture?
- may be inaccurate due to mouth flora
- suspect a pathogen if WBCs are present along with an overabundance pf 1 type of organism
- first morning sputum will represent deeper pulmonary secretions
- induce cough with nebs, pulmonary PT, aspiration, bronchoscopy
What should you always get with a sputum sample?
- always get a gram stain too!
- helpful for directing empiric therapy
What will you see in a atypical pneumonia sputum culture (mycoplasma and legionella)?
The sputum will contain abundant PMNs and few or no organism will be seen on the gram stain
What will you see on a sputum culture + with pneumococcal pneumonia?
abundant inflammatory cells and gram + diplococci
What are the indications for ordering a blood culture?
- bacteremia
- septicemia
- unexplained post operative shock
- unexplained fever of several days duration
- pneumonia
- chills and fever in pts with:
infected burns or UTIs
rapidly progressing tissue infections
post-op wound sepsis
indwelling venous or arterial catheters
-debilitated pts receiving:
abx, corticosteroids, immunosuppressives, parenteral hyperalimentation (nutrition)
When are blood cultures used to detect bacteremia (septicemia)
- should mainly be considered in pts w/ temp >101 suspected of having endocarditis or other systemic pathogens
- std of care for inpatients w/ pneumonia prior to starting abx
- can repeat in 1 hour or if fever
(transient bacteremia) - 2 specimens obtained from 2 separate sites 15 minutes apart (aerobic or anaerobic?)
- # of organisms / site is low
- helps id contaminants
- total: 30-40 ml of blood
Blood culture technique
- clean site first with 70% alcohol
- follow with butadiene x 2 minutes
- wipe bottle tops with alcohol
- go to lab immediately
- don’t draw blood cultures through central lines or IV (contamination)
How to collect a urine sample?
- clean catch, mid void specimen
- collect 5-50 ml of urine
- cath
- suprapubic tap
- best culture is in the morning (most concentrated)
Guideline of urine sample and transport
- midstream clean catch urine by pt
- need at least 1 ml
- transport a 4 degrees centigrade
- straight cath specimen: same parameters
- indwelling cath: disinfect port with alcohol
remove 5-10 ml of urine w/ needle and syringe -> transfer to sterile container, 4 C - suprapubic aspirate: at least 1 ml, at 4 C
- always send as UA and urine C & S
What does the urine dipstick test?
- pH
- specific gravity
- bilirubin
- protein (nephritis -> elderly)
- glucose -> diabetics
- ketones (diabetic ketoacidosis)
- blood: can’t always rely on
- nitrate: certain bacteria
- leukocyte esterase (pyuria)
indicators of possible pyuria
leukocyte esterase: if + on dipstick likely indicates pyuria
nitrite: indicates the presence of enterobacteriaceae that converts nitrate to nitrite
if neg but + symptoms of UTI still want urine culture
Microscopic analysis of urine
- eval of urine sediment from a spun sample
- presence of squamous epithelial cells = contamination from genital region
- normals:
rbc: 0-5 HPF
wbc: 0-5 HPF
bacteria: absent
casts: 0-4 hyaline LPF (kidney health)
crystals
Indications for a lumbar puncture
- look for blood (possible subarachnoid hemorrhage)
- find the organism causing the meningitis or brain abscess
- can do gram stain early on while awaiting culture reports
- cell and chemistry counts will aid in making the dx
CSF Culture
- suspected meningitis, high fever in infant
- lumbar puncture collect 1-5 ml of CSF
- transport at 25 degrees C
- gram stain and bacterial culture
- 4 tubes for: cell count, glucose and protein, gram stain and culture, and cell count to compare to tube 1
Pleural fluid culture
- thoracentesis
- dx and/or therapeutic: therapeutic- relieves dyspnea (breathe easier)
diagnostic: test the fluid for TB, CEA level (tumor marker), cytology, culture, gram stain, pH
Go under or over rib while doing a thoracentesis?
- over because nerves and vasculature along bottom of ribs
Synovial fluid eval?
- swollen joint
- infection, inflammation or uric acid
- meds shouldn’t be administered therapeutically at the same time because interfere with sample (corticosteroids, abx)
Body fluids collection technique
- peritoneal, pericardial, pleural, synovial
- aspirate using sterile technique into sterile syringe (usually pt has fever or elevated WBCs)
- remove needle and cap syringe to send to lab, sometimes put aspirate in a different container
- min. of 1-10 cc, send as much fluid as possible
- send at 4 degrees C
Genital tract culture for women
- swab cervix for gonorrhea, chlamydia as well as other organisms in suspected PID
- swabs of urethra or urine testing for chlamydia
- occasionally swabs of vagina are done
guidelines for vaginal/cervical culture
- no douching or tub bathing for 24 hours prior to collection
- swab -> checking for gonorrhea, chlamydia
When is a wet mount used?
- helpful in evaluating vaginitis
- saline wet mount of vaginal discharge can show epithelial cells covered with bacteria suggestive of bacterial vaginosis
- trichomonads
- can reveal multiple PMN’s
- KOH (K+ hydroxide) wet mount can reveal candida organisms
Swabbing male urethra indications
- special smaller swabs for urethral sampling for gonorrhea and chlamydia and other less common infectious agents
- urine samples for GC and chlamydia for nucleic acid amplification
- nucleic acid amplification more sensitive than culture
Urethral culture techniques
- usually on men
- culture for GC, chlamydia
- rapid enzyme test available
- Thayer martin/New York media
- anaerobic transport
Stool culture indications
- routine culture (for those with prolonged diarrhea or haven’t been on abx or hospitalized)
- > 2 grams, 4 degrees C
- C. diff toxin: test of choice for those who develop diarrhea on abx or in hospital > 3 days
- > 5 ml, 4 degrees C
Stool culture guidelines
- also check for ova & parasites
- usually look for staph, salmonella, shigella
- Don’t mix urine or TP with sample
- rectal swabs: worms
- ‘tape test’
