Laboratory Diagnosis of Infectious Disease

• What do you need to ensure if you're going to send a tissue sample?

  You send in correct transport medium

  
  

  You check patient data is correct

  
  

  If there is more than one sample you identify the sites of both/all

  
  

  You ensure that the pathologist has enough information as to where to sent the report back to

  
  

  You ensure that you understand & act on the result that is generated
• What is pathology/histopathology/cellular pathology?


  The study of disease & the process by which you obtain a diagnosis from a tissue sample
• What is cytopathology?


  Method of visualising cell structure where fluids, scrapings or aspirations are examined

  
  

  Provides predictive but not equivalent information to histology

  
  

  Quick, simple, cheap- serious complications are rare
• If you get a large sample that won't fit into one cassette, do you examine it as one piece?

  No, you'll most likely have to dissect it into smaller pieces to see what's going on in the different areas
• What needs to happen from the tissue in the pot to the tissue on the slide so that it can be assessed microscopically?

  You need to embed that tissue in a very hard and rigid substance (wax) so you can then attach that onto the microtone & use a very sharp blade to get the really fine sections

  
  

  Then put the very fine pieces of tissue into a warm water bath & dip a slide in- it's very easy to collect that fine slice of wax which contains the tissue and get the lab number immediately onto the slide so you don't lose identification

  
  

  Then, routine stain with HE
• How do you ensure that the wax is allowed to permeate and embed itself right into the tissues?

  Dehydrate the tissue by putting them into an overnight processor
• What steps are involved in a routine histological diagnosis?

  
• What extra work might need to be done when diagnosing from a slide?

  More H&E stained sections because:

  
  

  -cannot see a lesion that fits the clinical picture

  -biopsy difficult to interpret

  -being super cautious and checking there is nothing sinister

  -biopsy not put in the wax in the right way and the orientation of the tissue is making assessment difficult
• What are some special stains used in pathology and what for?

  PAS-fungi, salivary mucin/mucusGrocott-fungiElastin-blood vessel architectureZN (Ziel Neelson)-mycobacteria (TB)Gram-bacteriaTrichrome- collagen
• What is immunohistochemistry?


  Immunohistochemistry (IHC) refers to the process of selectively visualising antigens (e.g. proteins) in cells of a tissue section by exploiting the principle that antibodies bind specifically to antigens in biological tissues
• When is immunohistochemistry used?

  Tumour pathology:

  Different types of normal and tumour cells express different antigens

  Tag the antibody with a chromogen– reaction occurs on antigen binding and can be seen on tissue section

  Technique can be used in tissues that have been fixed in buffered formalin
• What are some examples of chromagens in immunohistochemistry and what tumours they would stain?

  
• How can you genetically test for tumours?

  Can dissect tumour cells from an unstained section taken from a paraffin block and use even more specialised techniques to look for chromosomal changes

  
  

  Not relevant for majority of tumours but a small proportion do have reproducible and single genetic abnormalities

  
  

  E.g. Ewings sarcoma has a reproducible translocation between chromosomes 11 and 22
• Are there any tests in which the biopsy should not be sent in formalin?

  Frozen sections- immediate intra-operative diagnosis

  
  

  Direct immuno-fluorescence

  
  

  Tissue to be banked for future research
• What is direct immunofluorescence?

  A special technique used to confirm the diagnosis of immunological diseases that can occur in the oral cavity

  
  

  Biopsy tissue sent in a special medium- Michel's medium
• What are some things that can go wrong in pathology?

  Mislabel specimen (wrong sticker)

  Don't label specimen at all

  Omit key clinical information or relevant medical history

  Omit information regarding who to send the report to

  Mix up of specimens in the laboratory- strict procedures
• Why might interpretation of the sample be difficult?

  Biopsy sampled is:

  -crushed-not sent in the correct medium (autolysis)-pathologist makes a mistake or mis-interpretation-typographical errors

  Responsibility lies with us all and with our supporting team to ensure that errors do not occur