ID- Lab Testing and Diagnostics Flashcards
Signal amplification has reduced false-positive results from cross contamination because the number of target molecules is not altered.
Consequently, the signal is directly proportional to the amount of target sequence present in the original specimen.
Lab procedures can be useful tools for three things
- Diagnosis of infection
- Assess the stage of infection or infectious process
- Evaluate HAI
What are the two key elements in selection and implementation of a test method?
- Accuracy
- Precision
What is accuracy and what two terms can be used to describe it?
Accuracy is the closeness of the result obtained to the true value.
1. Sensitivity
2. Specificity
What is sensitivity?
- The ability to detect all true cases of the disease
- The absence of of false-negatives.
#true positives/ (#true positives + # false negatives)
What is specificity?
The ability to correctly identify a negative result. The absence of false positive results.
# true negatives/ (True negatives + False positive results)
What is precision?
Precision= repeatability. When repeat testing of same sample produces same or similar results.
What is a proficiency testing program?
An external evaluation of the quality of a laboratory’s performance.
3 Common purposes for tests include:
- Screening
- Confirmation
- Diagnosis
What is screening used for?
Used to identify a disease process in individuals without signs or symptoms of the disease. In general they have high sensitivity, but require confirmation due to low specificity.
What is confirmation used for?
Used after a positive screen to ensure accuracy of the result and confirm presence of the condition.
Diagnosis
used for the evaluation of persons suspected of having a given disease state or characteristic.
How can definitive diagnosis be made (4)
- visualization of the agent in a specimen.
- Detecting specific products from an infectious agent (CDI Toxin)
- Detecting an immunological response (Ag-Ab) (Hep B surface antibody)
- Detecting presence of agent through nucleic acid hybridization and amplification techniques.
Direct Examination Methodologies include (3)
- Gram Stain
- Histology/Cytology
- Wet Mount
Antigen Detection
- Helpful for early diagnosis where cultures not yet positive, possible or practical.
- Direct method to test for infectious agents
- They differ from direct visualization in that they Include immunologic or serologic procedures
- Methods designed to detect entire agent (virus) or part of the agent (bacterial cell wall). (ELISA)
Antibody
Antibody immunoglobulin Produced by B-lymphocytes.
Facilitate the removal of the agent. Humeral.
5 Classes of antibodies- each with different properties and functions
Antibody Detection
Indirect Method by assessment of host response
Results may be qualitative (+/-) or quantitatively (titres)- should be interpreted in consideration of the predictive value of the specific test.
Consider historical infection.
Molecular Diagnostic Testing
subject of false +, sensitive to contamination
(PCR)
1. Target Amplification
2. Probe Amplification
3. Signal Amplification- signal directly proportional to the amount of target sequence present in the original specimen (reduced false +)
Tests for infectious process include:
- Body Fluid Analysis: total protein, specific gravity, cell count, boy fluid glucose, gram stain, culture.
- Cerebrospinal Fluid Analysis: color and clarity, protein, glucose, and WBCs.
What is the preferred method of differential cell count?
Centrifugation-> allows Wright’s staining of air dried cytospins and has good cell yield and preservation of cells.
C-reactive Protein (CRP)
Abnormal serum glycoprotein produced by liver during acute inflammation. Usually disappears quickly when inflammation subsides.
Sometimes used in diagnosis of meningitis, pneumonia, sepsis, TB, UTI.
Cold Agglutinins
Antibodies that cause clumping of type O red blood cells at cold temperatures. M. pneumonia, viral pneumonia, primary atypical pneumonia.
Liver Function Tests
Help aid in diagnosis of infectious processes such as chronic syphilis, subacute bacterial endocarditis, TB, inflammation due to certain viral infections (Hep b, C).
Arterial Blood Gas
Determines gas exchange across alveolar-capillary membrane in the lungs.
Worsening gas exchange may indicate inflammation in lung from bacterial or viral infection.
ABGs critical components in ID of HA pneumonias and ventilator associated pneumonias.
The WBC count is the total number of WBCs (leukocytes) in 1mm3 of peripheral blood
WBC count > 10 000 usually indicates infection, inflammation, or leukemic neoplashia.
Neutraphil
Primary function= phagocytosis.
Acute bacterial infections stimulate immature neturophilis (shift to left).
Appendicitis.
Lymphocytes
increases associated with pertussis, syphilis, toxoplasmosis.
Lymphocytes with frothy cytoplasm (atypical lymphs) associated with viral infections: cytomegalovirus, mono).
Monocytes
Increased monocytes commonly associated with with EBV, TB, subacute bacterial endocarditis, syphilis, protzoan.
Eosinophils
typically associated with allergic reactions.
May be increased in parasitic infections, leprosy, tb, systemic fungal infections, scarlet fever, chlamydial pneumonia in infancy.
Basophilia
Uncommon finding can be found in food/drug allergies, variola/varicella infections, ulcerative colitis.
Complete Blood Count: Absolute Neutrophil Count
Neutropenia is reduction of ANC to 1.5-2 x109/L.
ANC= Total WBCx ((Segs +Bands)/100)
Severe Neutropenia (agranulocytosis)= <0.5.
Determine whether an immunocompromised patient should be placed in protective isolation
Lymphocyte Subset : 2 types
T cells (mature in thymus)- involved in cellular-type immune reactions.
B cells (bone marrow) humoral activity- antibodies
T cells= killer cells
CD8= suppressor cells
CD4 Helper cells
Sedimentation Rate
Rate at which RBCs or erythrocytes fall. Lack sensitivity and specificity for disease processes; however, increased ESR rate associated with acute infection and inflammation.
Fecal Leukocytes
Determine whether diarrhea is invasive or non-invasive to the mucosa of the colon.
Presence of leukocytes indicates the cause of diarrhea is an organism or process, such as Salmonella, Shigella, Amoeba, Campylobacter, helicobacter, yersinia is breaking mucosal barrier of the colon.
Not common in viral enteritis or toxin-mediated diarrhea. CDI may or may not be positive.
Exotoxin
Proteins secreted by gram (-/+) (CDI, Clostridium tetani, Corynebacterium diphtheriae, staph aureus).
Many have A/B motif. A active portion, B binding portion.
Ability to lyse host cells, alter function of host cells, resulting in death of cell. Stimulates T-Cell responses in a cytokine cascade.
EIA and HPLC to detect.
Endotoxins
lipid A section of lipoolysaccharides that are only found on the outer membrane of some gram negative organisms (Ecoli, salmonella, shigella, pseudomonas, Neisseria, H Influenza, Bordetella pertussis, vibrio cholerae).
Cause damage to host through cytokine production by macrophages.
Limulus amebocyte lysate LAL test-> used to test water quality for hemodialysis.
Weil-Felix Agglutinin
To detect and differentiate rickettsial antibodies in serum.
Used to diagnose: Rocky Mountain spotted fever, Q fever, epidemic typhus, murine typhus, scrub typhus, rickettsialpox.
Urinalysis
Color, Clarity, presence of proteins, glucose, ketones, blood, nitrate, leukocyte esterase.
Dipstick: presence of leukocyte esterase and nitrite- screening tool for infection.
Nitrite- bacteria produce an enzyme that reduces urinary nitrates to nitrites.
leukocytes >10WBC/mm3 unspun or urine, or >5 WEBC high power field of spun urine. pyuria can be helpful when differentiating between colonization and true infection in the presence of bacteriuria.
