Diagnostic Testing Flashcards
What does anatomic pathology include?
Post Mortems
Histology
What does clinical pathology include?
Biochemistry
Serology
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Why do you have to be careful when selecting tests to be sent to NZVP or Gribbles?
It can be expensive!
Don’t want to waste the clients money.
e.g. full CBC costs $110 but the markup to clients can be up to $220
Why do you have to beware of test results and interpretations if your samples are sent to a medical laboratory?
- Medical pathologists aren’t usually trained to interpret animal samples.
- Their equipment is not set up for animal blood
- Reference ranges will generally be unreliable e.g. what may be abnormally high in people might be normal in a horse
- Some tests are completely unreliable in animals
- Haem techs won’t do manual blood smear examination
If you suspect an exotic disease in an animal e.g. mad cow disease. who do you contact?
MPI Biosecurity NZ
- They are part of the Investigation and disease centre (IDC)
- They will come and do everything for you at no cost to ensure it isn’t an outbreak
What information must you provide on the form when submitting samples?
- Signalment
- Clinical signs and history
- Any results of lab tests done in clinic
The more info the better!
Try to give them the big picture
What is important when vets are doing in-clinic tests?
Tests are only as reliable as the person who performs them
- should do that test at least once a week to ensure its fresh in head
- be under supervision of someone else
- It is better to do a few tests well than heaps poorly
What is Quality Assurance?
an overall set of protocols to ensure test results are valid
What is Quality Control?
Procedures set in place to ensure a manufactured product sticks to a defined set of quality criteria
What is Internal QC?
Control samples with known values are run with test samples at least once a week
What is external QC?
Duplicate samples are sent to the lab for comparison
Why is QC/ QA important?
Absolutely necessary for reliable results
- no QC = no guarantee that results are accurate
What is test selection?
Selecting tests that you know might help, confirm or exclude major DDx
What might occur if you only pick one lab test?
Might miss a vital piece of information
- tests used in isolation might be misleading because the results might be different tomorrow or might have been different yesterday
Why might repeat sampling be necessary?
To confirm suspicious results or to assess to progress in an animal
- this is particularly important in endocrine cases
What units do NZ labs use?
S.I units
- ensure this is interpreted correctly
US publications still use US customary units
What must you consider when interpreting results (bell curve)?
The reference range is the mean +/- 2 standard deviations.
- Thus 5% of healthy animals fall outside the reference range - just by chance
What do reference intervals represent?
The distribution of test results in a healthy population
What are the measures of how well tests predict the actual disease state of the animal?
- Sensitivity/ Specificity
- Accuracy
- Predictive Values
What is test sensitivity?
The frequency that a test is positive in patients that have the disease
- girl that cries about everything whether it’s actually worth crying about
TP / TP+FN x 100
High sensitivity is good for ruling diseases out - low false negatives
SN-OUTs
What is test Specificity?
The frequency that a test is negative in patients that do not have the disease
- Boy that doesn’t like peas - picks every single one off his plate without missing any
TN / TN + FP x 100
High specificity is good for ruling a disease in - low false positives
SP-INs
What is diagnostic Accuracy?
The frequency that a test correctly classifies an animal as having or not having the disease
TP +TN/ total # x 100
What is a Positive Predictive Value (PV+)
The probability that a positive test result indicates the animal has the disease
TP /TP + FP x100
(TP / all positive results)
What is a Negative Predictive Value (PV-)?
The probability that a negative test result indicates the animal does not have the disease
TN / TN + FN x 100
(TN / all negative results)
If a test has a low specificity (44%) what does this mean for the test?
It is not good at ruling things in
- there are %56 false positives but these animals actually do have the disease
If a test has high sensitivity (98%) what does this mean for the test?
It is very good at ruling things out
- only 2% false negatives
- generally if the animal has the disease it will test positive - rarely misdiagnoses those with the disease
If a test has high specificity (92%) what does this mean for the test?
It is very good at ruling diseases in
- only 8% false positives
- rarely gives positive results in healthy patients
If a test has low sensitivity (35%) what does this mean for the test?
It is not good at ruling things out
- there are 65% false negatives
Why is it important to know the strengths and weaknesses of tests?
- Every test has them!
- Knowing the strengths and weaknesses of a test influences your interpretation of test results
- Don’t always need to know sensitivity and specificity - just need to know how good the test is!
PCV test
- Diagnoses and monitors anaemia
- Assesses the need for blood transfusion
- Monitors fluid therapy
- Examining tubes after centrifugation can give useful information
Plasma Protein
Measure plasma from PCV tube using refractometer
- Differentiates blood loss from haemolytic anaemia
- Monitors fluid therapy
- Detects hypo and hyperproteinaemic diseases
Urinalysis
Has three components:
- Specific Gravity
- Chemistry
- Sediment Exam
Measured using refractometer
- Assesses renal concentrating ability
- Assists in interpretation of urine chemistry results
- SG varies in normal animals depending on state of hydration (1.008-1.012
Urine Chemistry
Measured by dipsticks
- Ignore nitrate. leukocytes and SG
Interpretation depends on [urine] as determined by refractometer
- +ve blood could indicate HB, intact RBCs or myoglobin
Urine sediment
Check for casts, crystals, cells, microorganisms
- Requires some experience and training
Blood Glucose
- Easily performed in clinic
- Useful for diagnosis and monitoring diabetics
Faecal Blood
Kits available for detecting small amounts of blood in faeces
- False positives occur in animals on meat diets
Faecal Smears
Significant numbers of neutrophils may indicate bacterial disease of large colon or ulcerative colitis
Eosinophils may suggest allergic or parasitic disorders
Skin Scraping
Best performed in clinic because repeat sampling is usually needed
- Demodex and Sarcoptes easily recognised if present
Cytology
Basic Interpretations of: ear swabs, effusions, joint fluids and skin lumps
- refer difficult cases to lab
