Diagnosis of Infectious Diseases

From Physician to Laboratory

• Techniques used to characterize infections depend on the clinical syndrome and infectious agent
• No single test can isolate or characterize all potential pathogens
• Clinical information is more important for diagnostic microbiology compared to clinical chemistry or hematology

Diagnosis of Infectious Diseases

• Proper diagnosis requires patient history
• Proper diagnosis requires a physical examination
• Proper diagnosis involves careful evaluation of the patient's signs and symptoms
• Appropriate laboratory tests and other methods are needed for diagnosis
• Proper selection, collection, and transport of clinical specimens is necessary

Guidelines for Specimen Collection and Transportation

• Collection of the specimen before antimicrobial administration is essential
• Prevention of contamination with external organisms or normal flora is key

Three Components of Specimen Quality

• Proper specimen selection ensures the specimen type is appropriate for diagnosing the suspected infectious disease
• Proper collection eliminates or minimizes contamination; special collection devices may be needed
• Proper transport maintains viability and preserves agent morphology; rapid transport with ice or preservatives may be needed

Clinical Specimen Requirements

• Collect the appropriate specimen type: swab, sputum, urine, blood, biopsy, nail/skin scraping or broncho-alveolar lavage
• Collect from a site where the pathogen is most likely to be found
• Collect before antimicrobial therapy begins
• Collect during the acute stage for best results
• Avoid harming the patient or causing discomfort during collection
• Collect a sufficient quantity of the specimen
• All specimens should be placed or collected into a sterile container

Quality Assurance (QA)

• QA continuously identifies, monitors, evaluates, and improves reliability
• QA ensures the final results reported by labs are correct
• QA ensures right test is carried out on the right specimen
• QA ensures the right result and right interpretation

Quality Control (QC)

• QC monitors accuracy, reliability, and reproducibility of tests
• QC identifies and corrects problems

Components of a CML's Quality Control Program

• Standard operating procedures manual includes procedures written for every aspect of CML work
• Test verification uses methods for most accurate results
• Test methods and procedures: the QC program must monitor, document, and evaluate all aspects of every test procedure performed
• Media, reagents, and staining solutions need to avoid expiration dates
• Equipment and instruments should be maintained and monitored for equipment performance
• Records and reports should include QC measures
• Proficiency testing uses positive and negative controls for every test procedure

Criteria for Specimen Rejection

• Missing or inadequate identification
• Insufficient quantity
• Collection in an inappropriate container
• Suspected contamination
• Inappropriate transport or storage
• Unknown time delay
• Hemolysed blood sample

Clinical Specimens from Various Anatomical Sites and Organ Systems

• Circulatory System: Blood (usually sterile); samples taken for septicemia/bacteremia if suspected
• Skin, Abscess, and Wound Specimens: Tissue biopsy/swab
• Eyes and Ears: Swab/aspirate/scrapping
• Respiratory System: Sputum/swab/broncho-alveolar lavage/aspiration
• Central Nervous System: CSF/blood culture
• Urinary Tract: Urine analysis/culture
• Genital Tract: Urethral/vaginal discharge/blood
• Oral Cavity: Oropharyngeal swab/blood
• Gastrointestinal Tract: Blood/stool/gastric lavage
• Body Fluids: Pericardiocentesis/thoracocentesis/paracentesis

Obtaining Skin Specimens

• For pustules, remove covering and collect fluid and basal cells
• For petechiae, vigorously scrape the outer margin of the lesion
• Collect abscess contents via aspiration using a needle and syringe
• For wound specimens, collect sample via aspiration or from the advancing margin of the lesion

Factors Affecting Quality Assurance (QA)

• Analytical proficiency of personnel is key
• Documentation of policies, plans, procedures & instructions
• Specificity & sensitivity of tests
• Equipment reliability to meet technical needs
• Reagent stability/efficiency
• Use of internal/external controls
• Standard operating procedure to perform a routine activity

Factors Limiting Usefulness of Microbiology Investigations

• Wrong sample (e.g., saliva instead of sputum)
• Delay in transport/inappropriate storage
• Overgrowth by contaminants e.g. blood cultures
• Insufficient sample/sampling error
• Problems specific to mycobacterial disease
• Failure to take the specimen from infection site
• Sample was not taken aseptically and if sample size is inadequate
• Failing to maintain metabolic requirements for sampling, storage, and transport

Laboratory Investigation of Microbial Infections

• Microscopy
• Culture techniques
• Biochemical tests
• Serological identification
• Chromatography
• Molecular biology techniques
• Bacteriophage
• Animal pathogenicity