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Principles of diagnostic methods

This table gives you an overview on diagnostic targets and the most important methods with their advantages and disadvantages.

Diagnostic targetMethods of choicePros and cons
   
Microscopic detection of whole parasites in blood, faeces, urine or tissues (protozoa, helminth ova or larvae)
  • A variety of concentration methods and special staining procedures (given later in this chapter)
  • Diagnostic skills and microscopic equipment needed
  • Sensitivity not always optimal (need to examine several specimens spaced in time)
  • Specificity depends on microscopist
  • Methods fail during prepatent helminth infections and if humans are intermediate hosts of cestodes
   
Parasitic DNA or RNA
PCR (Polymerase chain reaction) or nested PCR
  • Conventional PCR
  • Real-time PCR
  • Nucleic Acid Sequence-based Amplification (real-time NASBA)
  • Methods with highest sensitivity and excellent specificity
  • Some false negative results due to inhibitors in sample
  • Time consuming procedure (except real time PCR and NASBA)
  • Investment in equipment and rather expensive reagents
   
Direct detection of Parasite antigens in blood, stool or urineImmunoassays with mostly specific monoclonal antibodies
  • Immuno-enzyme methods (e.g. ELISA)
  • Direct fluorescent antibody tests
  • Immuno-chromatographic assays (“Rapid diagnostic tests”)
  • High sensitivity
  • Detection of active infections
  • Efficient testing of many samples
  • False negatives due to inhibitory host antibodies
   
Immune response against
Parasite antigens
(Detection of antibodies in serum)
Various indirect Immuno-assays using labelled anti-human immunoglobulin conjugates
(e.g. Enzyme-Immunoassays, Indirect Fluorescent Antibody tests, Western blots and others)
  • High sensitivity
  • Limitations in specificity (due to crude antigen preparations)
  • Limitations in sensitivity (when recombinant antigens or synthetic peptides are used)
  • Early diagnosis before patent period (helminth infections)
  • Persistence of host antibodies after cure