Lab Medicine Flashcards
When would you want to order a blood culture?
When you are seeking out systemic infection
What is the gold standard for diagnosing strep throat?
A throat culture
What is the specific organism responsible for strep throat?
Streptococcus pyogenes
How long does it take for throat culture results to come back?
24-48 hours
Other than for strep throat, when do you want to order a throat culture?
When diphtheria, gonococcal pharyngitis, or thrush (Candida) is suspected
Why would you not do a gram stain on a throat swab?
Because it is impossible to visually distinguish between normal flora streptococci and strep pyogenes
When would you do a sputum culture?
When pneumonia or TB is suspected
If you suspect pneumonia or TB, but the patient can’t cough, how do you get a sputum culture?
- Induction of sputum
- Transtracheal aspirate
- Bronchial lavage
- Lung biopsy
What is the most common cause of community acquired pneumonia?
S. pneumoniae
What organism is the sputum gram stain not sensitive or specific for?
S. pneumoniae
When is the sputum gram stain useful?
In broadening initial coverage in patients to be hospitalized for CAP
When do you order a bacterial wound culture?
- To determine whether a wound is infected
- To identify the bacteria causing the infection
- To prepare for susceptibility testing when required
- To determine whether wound treatment was effective
Are gram stains performed on a wound culture?
Yes; but be aware that no bacteria on stain does not rule out a wound infection
What are the pathogens that frequently cause abscesses of the brain, lungs, and abdomen?
- Anaerobes (bacteroides fragilis)
- Gram positive cocci (staph aureus, strep pyogenes)
What are the pathogens that frequently cause traumatic open wound infections?
Soil flora (eg. clostridium perfringens)
What are the pathogens that frequently cause surgical wound infections?
Skin flora (eg. staph aureus)
What are the pathogens that frequently cause infections from bites?
Dog or cat bites: pasteurella multocida
Human bites: mouth anaerobes
Are aerobes or anaerobes more frequently involved in wounds?
Anaerobes; so when culturing, make sure to place specimens in proper collection tubes and promptly transport them to the lab!
Since wound infections are frequently due to multiple organisms, what should you do when culturing?
Culture specimen on different media under different atmospheric conditions
When should you perform a CSF culture?
When meningitis is suspected
What are the most important causes of acute bacterial meningitis?
Three encapsulated organisms
- Neisseria meningitidis (meningococcal)
- Strep pneumoniae (pneumococcal aka streptococcal)
- H. influenzae
In what cases might you get negative CSF cultures?
Encephalitis, brain abscess, subdural empyema
What guides the empirical treatment of acute bacterial meningitis?
Gram-stained smear of the sample
What is the most rapid way to detect the pathogenic organism involve in acute bacterial meningitis?
Immunofluorescence
What are the most common organisms involved in subacute meningitis?
- Mycobacterium tuberculosis
- Cryptococcus neoformans (a fungi)
How do you detect cryptococcus neoformans in spinal fluid?
Use India ink!
What are special steps that must be taken in the case of suspicion of tuberculosis meningitis?
- Acid fast stains should be performed’
- Fluid should be cultured and held for a minimum of 6 weeks
- M.tuberculosis can be present in small numbers
When should a urine culture be obtained?
-When pyelonephritis or cystitis are suspected
What is the most frequent cause of uncomplicated UTI?
Escherichia coli
Other than E.coli, what are other common agents of UTI?
Enterobacter, Proteus, Enterococcus faecalis
Since urine acquires organisms of natural flora as it passes through the distal portion of the urethra, what are some tips to avoid obtaining these organisms in your urine culture?
- Clean midstream catch
- Catheterized sample
- Suprapubic aspirate
What must be found to diagnose significant bacteriuria in asymptomatic persons?
Bacterial count of 100,000/mL from a urine sample
What must be found to diagnose significant bacteriuria in symptomatic persons?
Bacterial count of 100/mL from a urine sample
What kind of tests look at the efficacy of antibiotics against the growth of an organism?
Sensitivity tests
How do you diagnose viral diseases using clinical specimens?
5 approaches:
- Cell culture
- Direct microscopic identification
- Rise in antibody titer or presence of IgM antibody
- Viral antigens in blood or body fluids
- Viral nucleic acids in blood or the patient’s cells
The growth of viruses requires what kind of cultures?
Cell cultures! Because viruses only replicate in living cells, not on cell-free media
What is CPE?
A change in the appearance of the virus-infected cell
Virus growth in a cell culture frequently produces a CPE that can provide a presumptive ID
How no CPE is elicited on a cell culture, how is viral presence detected?
Direct microscopy (among other things)
What are the two types of fungi?
Yeasts and molds
How do yeasts grow?
As single cells that reproduce by asexual budding
How do molds grow?
As long filaments (hyphae) that form a mat (mycelium)
- Some hyphae form transverse walls (septate)
- Other hyphae do not form walls (nonseptate)
What quality do many medically important fungi share?
They are thermally dimorphic! Aka, they form different structures at different temperatures
- They exist as molds in the environment at ambient temperature
- They exist as yeasts in human tissues at body temperature
How do you identify fungal diseases?
Four approaches
- Culture of the organism
- Direct microscopic examination
- DNA probe tests
- Serologic tests
What kind of agar are fungi frequently cultured on?
Sabouraud’s agar
What is so special about Sabouraud’s agar?
- Facilitates appearance of slow-growing fungi for inhibiting bacterial growth
- Supports fungal growth for several weeks
For what types of specimens is direct microscopic examination ideal for fungal identification?
-Specimens such as sputum, lung biopsy material, and skin scrapings (depend on finding characteristic asexual spores, hyphae, or yeasts in the light microscope)
What distinguishing feature do you see during direct microscopic examination of cryptococcus neoformans from CSF with India ink preparations?
The wide capsule
What is the benefit of using DNA probes in fungal identificaiton?
-Can identify colonies growing in culture earlier than visual detection, and diagnosis can be made more rapidly
What is the goal of serologic tests in fungal identification?
- Tests for the presence of antibodies in the patient’s serum or CSF
- Useful in diagnosing systemic mycoses
What are two bacteria that are acid fast?
Mycobacteria
Norcardia asteroides
What are mycobacteria?
Aerobic, acid-fast bacilli (rods)
What are the major mycobacteria pathogens?
- Mycobacterium tuberculosis
- Mycobacterium leprae
What are atypical mycobacgteria?
- Mycobacterium avium-intracellulare complex
- Mycobacterium kansasii
Can cause tuberculosis-like disease; less frequently pathogens in immunocompromised
Instead of gram stained sputum, what are more often used for specific diagnosis of mycoplasma?
Serologic testes
What is the Cold Agglutinin Test and what is it used for?
- A mycoplasma serology test
- Patients with certain illnesses develop autoimmune antibodies that agglutinate human red blood cells in the cold (4C) but not at 37C
In what diseases have cold agglutinin antibodies been observed?
- M.pneumoniae
- Adenovirus
- Infectious tuberculosis
When is a stool culture performed?
Primarily for cases of enterocolitis
What are the most common bacteria causing diarrhea in the US?
- Shigella
- Salmonella
- Campylobacter
- (not as common, but E.coli strains also important)
What does a methylene blue stain that reveals many leukocytes in a stool culture indicate?
An invasive organism rather than a toxigenic one
Is a gram stain routinely done from a stool culture?
No; large numbers of bacteria in the normal colon flora make the interpretation difficult
How is intestinal amebiasis diagnosed?
By finding either trophozoites in diarrheal stools or cysts in formed stools
How many stools for ova/parasites should be examined when cysts are suspected?
At least 3, since cysts are passed intermittently
What does a complete examination for cysts include and what is the purpose?
- Wet mount in saline
- An iodine-stained wet mount
- A fixed, trichrome-stained preparation
Each of these steps brings out different aspects of cyst morphology; helps distinguish amebic from bacillary dysentery
What is seen in bacilliary but not in amebic dysentary?
Inflammatory cells such as PMNs seen in bacilliary, but not in amebic dysentary
How is giardia diagnosed?
Direct microscopy
ELISA test that detects a giardia cyst wall antigen in the stool is also useful
When would you do a string test, aka an Enterotest?
When symptoms indicating giardia persist but the direct microscopy and ELISA tests are negative
How do you perform a String test (aka the Enterotest?
- Swallow a gelatin capsule attached to a long string
- Tape string to your cheek and leave in place 4-6 hours or overnight
- Withdraw the string and examine under the microscope
- Trophozoites adhere to string and can be visualized
How do you identify malaria?
Thick and thin blood smear; diagnosis is made by visualizing the characteristic organisms
What is the most frequently used nucleic acid amplification test (NAAT)?
PCR! Aka polymerase chain reaction
What is PCR used for?
To detect microbiologic organisms and genetic diseases
What does PCR do?
It amplifies the normal DNA replication process; genetic material is identified by agarose gel electrophoresis
What is a potential drawback of PCR?
Contamination with extraneous DNA
What is the ligase chain reaction test?
A new type of NAAT that has greater specificity than PCR
What does the ligase chain reaction test detect?
-C.trachomatus and N.gonorrhoeae in urine
What is different about identifying syphilis?
-Treponema pallidum can not be grown in culture in the microbiology lab
How do you detect treponema pallidum?
- Detect spirochete by direct microscopy
- Detect specific antibodies in serum
- Dark field microscopy using dark-field condenser looking for motile spirochetes
What are the most useful tests for syphilis screeening?
VDRL and RPR
What are VDRL and RPR measuring and what is their role in identifying syphilis?
- Measure non-treponemal antibodies
- Non specific, 1-2% of time false-positives occur
- False negatives can happen early in disease because it takes several weeks to develop antibodies
Once you get a positive result of syphilis in the VDRL or RPR, what is the next step to confirm results?
Positive results need to be confirmed with FTA-ABS or MHA-TP
What is the FTA-ABS?
A fluorescent treponemal antibody absorption test to detect treponemal antibodies
-Used to confirm syphilis screening tests
What is the MHA-TP?
Microhemagglutination, a serum test to dect treponemal antibodies
-Used to confirm syphilis screening tests
What is the initial screening test to detect HIV antibodies and what happens if you get a positive test?
ELISA (enzyme-linked immunosorbent assay)
- Can detect HIV-1 and HIV-2
- High sensitivity and specificity, though false negatives can false-positives can occur
- Need a confirmatory test for diagnosis
What is the confirmatory test for detecting HIV antibodies?
-The Western blot!
Why is the Western blot not good for screening for HIV?
-Lower sensitivity, but highly specific
What are the two STIs that require both an initial screening test and a confirmatory follow up test?
Syphilis identification and HIV identification
