Diagnosis of Infectious Disease Flashcards Preview

Micro/Immuno Part 2 > Diagnosis of Infectious Disease > Flashcards

Flashcards in Diagnosis of Infectious Disease Deck (30):

clinician needs to know

-labs test menu
-specimen collection and transport guidelines
-testing policies
-appropriate specimens and tests
-how to prioritize


diagnosis of infectious disease

-clinical syndromes are rarely specific for single pathogens
-having an understanding of the organisms most frequently associated with a clinical presentation allows for empiric therapy until definitive lab results are available


fiver general approaches for diagnosis

-detection of bacterial antigens
-demonstration of specific nucleic acids (molecular techniques)
-detection of antibodies directed against the organism (serology)


typical turnaround times using conventional clinical microbiology techniques

-day 1-specimen processing, plating and staining
-day 2- culture exam, identification and sensitivity setup
-day 3- id and sensitivity tests read
-day 4 physician review of culture results

-certain organisms take longer to grow and therefore culture results may take longer
-fastidious-2-4 weeks
-fungi-4-6 weeks
-TB up to 8


garbage in garbage out

-proper collection and transport is critical to the quality of results produced by the lab
-validity of all diagnostic information produced in the lab is contingent on the quality of the specimen received
-consequences of poorly collected and/or poorly transported specimens include failure to isolate the causative org, and recovery of contaminants or normal flora, which could lead to improper treatment of the patient


fundamentals of collection

-collect specimen in acute phase of infection-before antibiotics
-select correct anatomic site
-use proper technique-minimal contamination
-collect appropriate quality
-pack to maintain viability and prevent leakage
-label specimen accurately
-transport or store specimen promptly


various stains and prep

-confirm submitted material is representative
-identify cellular components and debris of inflammation to estimate the probability of infection
-identify specific infectious agents
-guide physicians to early treatment with antibiotics
-support or refute initial physician idiagnosis


gram stain

-cell wall characteristics
-shape and arrangement allow for presumptive ID
-demonstrates cellular material and inflammatory response (WBC)



-s aureus or s pyogenes
-organism gram reaction and morphology provides possible cause allowing for selection of empiric therapy
-the presence of inflammatory cells is also indicated


common things happen commonly

-knowledge of the most frequent agents associated with specific infections and their gram reaction and morphology can allow for presumptive id before culture or other test results are available


bacterial meningitis in newborns

-e coli- GNR
-group B- GPC chains
-listeria GPR


bacterial meningitis in children

-pneumococcus- GPC chains
-niesseria- GNDC
-H influenzae- pleomorphic GNR


detection of bacterial antigens

-variety of immunoassays are available for the detection of bacterial antigens directly from clinical material
-generally 100% specific but not 100% sensitive
-decreased sensitivity may be due to improper specimen collection and transport, inappropriate timing of collection, or assay format
-in some cases, antigen detection is the method of choice-legionella in urine


antigen detection formats

-latex agglutination-antibody coated onto latex particle, when a sample contains the antigen it causes visible agglutination of the particles
-coagglutination-antibodies are bound to bacteria, wen sample contains antigen it causes visible agglutination of the bacteria
-direct fluorescent antibody-antibodies tagged with fluorescent dye
-Immunochromatographic assays- sample mobilizes gold particles coated with monoclonal antibodies. if antigen is present in the sample, a complex is formed between the capture antibody and the monoclonal antibody gold conjugate, which can be seen (gold+monoclonal binds to capture antibody and antigen)


Rapid diagnosis of group A strep

-a number of assays are available that allow for the detection of S pyrogenes directly from a swab of the tonsillar area
-not 100% sensitive 100% specific


serological diagnosis

-not all infectious agents have available antigen assays or culture techniques
-determines disease susceptibility or immunity
-diagnose a current or previous infection
-Ad-specimen easy to get, tests widely available, ease of specimen transport
-Disad-IgG requires acute and convalescent sera in some diseases, IgM can have false pos and false neg, 2-3 week delay in diagnosis in infections with short incubation period


IgM after infection

-appears in serum in 1-2 weeks
-persists for 2-3 months
-consistent with current or recent infection


IgG after infection

-appears 2-3 weeks after infection
-may persist for life
-may represent somewhat recent infection or immunity


antibody titers

-amount of antibody at a particular dilution of patient serum
-1:2, 1:4 etc
-higher dilution is consistent with higher level of antibody in patient serum


acute and convalescent antibody titers

-asses IgG at 2 phases
-simultaneous assay of both sera is most meaningful
-acute phase-first serum sample collected after exposure or symptom onset, may be asymptomatic
-convalescent phase-serum sample collected 2-4 weeks later
-compare acute and convalescent results-4 fold rise in titer of paired sera collected 204 weeks apart verifies recent infection and is diagostically significant


molecular approaches

-nucleic acid based tests for nucleic acid amplification tests are increasingly used for detection and id of microorganisms and viruses directly from clinical specimens, id from culture material
-only a few FDA approved


why use molecular test

-allows for detection and id of causative agents that are:
-difficult to culture-metapneumovirus
-slow growing
-highly infectious-brucella
-increased sensitivity over current methods-flu A and c diff
-detect carrier/colonization-MRSA
-detection of resistance



-herpes simplex encephalitis
-most common cause of sporadic, fatal encephalitis
-infection of brain parenchyma, esp temporal and frontal lobes-hemorrhage and necrosis
-occurs as a complication of primary infection in the neonate-HSV2
-reactivation of latent disease in older children and adults-HSV1
-clinical presentation and imaging are sensitive but not specific
-mortality is >70% if untreated
-Upstate has RT-PCR for detection and uses FRET probe and melt curve analysis to distinguish strains for treatment
-could also do DFA or culture


limitations of nucleic acid amplification tests

-relatively expensive-reagents, equipment, space
-availability-few FDA cleared assays
-still need for organism isolation/AST
-detect both live and dead organisms
-risk of contamination and false pos
-not 100% sensitive or specific


culture and recovery

-traditional method
-organisms have specific nutritional and growth requirements
-clinical labs use a variety of media to enhance the recover
-cannot culture for all organisms all the time- need to ask for things that aren't routine


initial differentiation based on growth characteristics

-initial colony observation allows for preliminary id based on size, topography, opacity, and different reactions based on agar


lactose fermenting GNR

-e coli,


non-lactose fermenting GNR



viral culture

-tissue culture-need to grow human cells on plates for viruses
-some won't grow


film array

-multiplex PCR to detect many organisms
-can detect what virus from one sample