Introduction To Diagnostic Testing Flashcards
What is the diagnostic process?
- History (PC, HPC, PMH, ADT, FH, SH, ROS)
- Examination (general, then click into system)
- Differential diagnosis
- Investigations (biochemistry, microbiology, haematology, histopathology)/imaging
- Diagnosis
- Treatment
What are the 5 uses of investigations?
Diagnosis Suitability for treatment Prognosis Monitoring of treatment Screening
How can you ensure you use investigations appropriately?
Must be relevant, accurate + used within clinical context (must be provided with requests + considered when interpreting results)
Irrelevant investigations lead to unnecessary risk, cost + false positive results
When can errors occur in investigations? Give 1 example of each.
Before (e.g. labelling error)
During (e.g. equipment faulty)
After (e.g. interpretation flawed)
When and why should you request an investigation?
Should be requested after clinical assessment of patient (history + exam)
Information you gain from result must outweigh risk to patient from investigation
In order, list what investigations you would start with and lead up to. Why?
Start with least invasive + risky and move up e.g. urine sample -> blood test -> CT scan -> biopsy
What are the main biochemical tests you can run?
Liver Function Test (LFT)
Urea + Electrolytes (U&Es)
What do Liver Function Tests (LFTs) show you?
Many blood proteins synthesized in liver so a blood sample can be tested for various proteins to assess liver function
Used to: give information about liver disease, involvement of liver in other disease, medication effects, to distinguish between liver disorders + to assess extent of damage
How would you distinguish between a LFT result showing a liver functionality or structural issue?
Functional: proteins synthesized my liver decreased in blood so its not functioning properly
Structural: increase in protein that is part of the liver structure itself
What 5 proteins can LFTs test for?
Albumin: major component of total blood protein, MADE in liver, decreased in chronic liver disease
Aminotransferases: found in liver + others, high levels indicate liver damage
Bilirubin: breakdown product of haem, usually cleared by liver so elevated levels suggest liver disease
ALP: found in liver + others, blood levels increased in acute viral hepatitis
Y-GGT: found in liver + others, blood levels high in hepatitis + chronic liver disease
Why do we test Urea & Electrolytes (U&E)?
Urea: made in liver + removed from blood by kidneys so increased levels suggest kidney problems
Electrolytes e.g. Na, K, Cl + HCO3 can be imbalanced for many reasons linked to blood pH; can be used to monitor treatment effects
What is involved in a FBC in haematology tests?
Hb: [Hb] in blood; lowered in anaemia
haematocrit/PCV: proportion of blood volume made up of cells
RCC: estimated no. of RBCs, can differentiate between anaemias
MCV: average volume of RBCs; change of size can differentiate between anaemias
MCH (amount)/ MCHC (concentration)
WBC/platelets
What types of anaemias can Mean Corpuscular Volume (MCV) differentiate between?
Microcytic and macrocytic; decreased oxygen level in both
What is histopathology? What can it detect?
Microscopic examination of tissue from biopsy, surgery or autopsy for signs of disease; can detect cellular changes e.g. metaplasia + neoplasia & increased presence of cells not normally high in tissues e.g. immune cells
How does histopathology work?
Tissue sample is cut, fixed + stained before viewing under a microscope
Most common stain is haematoxylin + eosin (H&E) which will stain pink
What are the 3 types of microbiology samples that can be examined?
- Swabs e.g. pus, skin, nose, throat, urethra, vagina etc.
- Body fluids e.g. pus, urine, faeces, blood, CSF etc.
- Body tissues e.g. biopsies (rarely)
What are the direct microbiology techniques?
M, C + S (bacteria or fungi)
Antigen detection tests (e.g. malaria, legionella + C.Difficile toxin)
PCR tests (e.g. viruses, mycobacteria, 16S ribosomal DNA)
What are the indirect microbiology techniques? Why are they called indirect techniques?
Serology tests e.g. rare of neglected tropical infection
Indirect as they are detecting bodies RESPONSE to an infection, not the actual infection like direct tests will be
What can M, C + S be used for?
Mainly bacteria or fungi
Viruses - electron microscopy + viral culture (rarely done)
Protozoa - light microscopy (c + s rarely done)
What is done in M, C + S tests?
M: uses specific stains e.g. Gram
C: media selective for certain bacteria
S: done from discs on culture (smaller the zone of inhibition, the less sensitive the bacteria is to antibiotic)
Biochemical profiling of isolates gives more precise identification
What is a antigen?
Biochemical molecule able to raise an immune response
What are antigen detection tests?
Some antigens highly specific to a particular pathogen so detection of them can make a good diagnostic test
Can be detected via monoclonal antibodies specific to antigen in tests
When are antigen detection tests useful?
For rapid + early detection of pathogen when other techniques not feasible
The underlying methodology of antigen detection tests is similar to that of ____ __.
Pregnancy tests
What is PCR?
Exponential replication of specific DNA/RNA sequences some of which are highly specific to particular pathogen therefore, detection of them can make a good diagnostic test
What is the step-by-step process of PCR?
- Target DNA/RNA sequence heated up + denatured so 2 strands are pulled apart
- Complementary primers added + anneal to target sequence
- DNA polymerase added to extend primers creating 2 new double helices with same start sequence
- These steps are continuously cycled through where each cycle doubles the original target sequence
- Now there are lots of copies so easy to detect/visualise using normal techniques e.g. fluorescent dies that bind DNA/RNA
When is PCR useful?
PCR is highly specific + sensitive so its useful for rapid + early detection of pathogens especially when small numbers of organisms present or when other techniques not feasible
What is serology tests?
Detection of Abs against a specific pathogen
Specific Abs raised by specific Ags on certain pathogens so detection of these antibodies can make a good diagnostic test
Specific Ags easily produced from pathogen + used in serology tests for that pathogen
E.G. ELISA, CFT
What are the strengths + weaknesses of serology tests?
Strength: easy to manufacture tests in various formats + can be effective so long as Ab response is produced
Weakness: Cross-reacting Abs may reduce specificity + Abs take 7-10 days to appear after infection
What is Enzyme-linked immunosorbent assay (ELISA)? How does this serology test work step-by-step?
- Ag added to plate
- Patient sample with Ab in it added + Ab binds the Ag
- HRP-conjugated detection Ab (secondary Ab) added which binds the Abs in patient sample
- TMB added
- HRP converts TMB to TMB chromagen which is blue + can be visualised
- If colour is present, pathogen Abs are present in patient sample
How are references ranges made for investigations?
95% of normal values lie within 2 SDs of the mean so this is normally defined as the normal range for investigations
Why is calculating the mean average of limited value in reference ranges for investigations?
Because normal values vary across populations with a normal distribution so someone could deviate from mean + still be healthy for example.
