Introduction to Clinical Pathology

Question 1

What is clinical pathology?

Answer 1

study of disease in the clinical environment by the use of laboratory assays

pattern recognition!

Question 2

What are the main 5 main tests done in clinical pathology?

Answer 2

1. hematology - CBC, blood smear examination
2. clinical chemistry - biochemistry profile
3. coagulation tests
4. blood gas analysis
5. endocrinology

Question 3

What is characteristic of a valid test?

Answer 3

measures the parameter (analyte) of interest over a range of values with minimal interferences

• abnormal values should have a strong association with a disease or condition with few false positive and negative results

Question 4

No laboratory test is perfect. What is done to overcome this?

Answer 4

several tests are often used in combination to diagnose or categorize a disease

Question 5

What do test sensitivity and specificity depend on?

Answer 5

prevalence of a disease

NO test has 100% sensitivity and 100% specificity
(>70% is pretty good, there is always a margin of error)

Question 6

What is test sensitivity? What happens if the sensitivity of a PCR for lymphoma is 91% and 100 animals with lymphoma are tested?

Answer 6

ability of a test to detect patients who truly have the disease (true positives) and rules out false negatives

• 91% have a positive result
• 9% have false negatives

Question 7

In what tests is it most important to have high sensitivity? Why?

Answer 7

screening tests

negative result with a highly sensitive test will effectively rule out a disease

tests with high sensitivity are best used to rule OUT disease (SnOUT)

Question 8

What is test specificity? What happens if the specificity of a test is 95% and 100 animals are tested?

Answer 8

ability of a test to detect patients that truly do not have a disease (true negatives)

• 95% of animals will have negative results
• 5% of animals will have false positive results

Question 9

What happens when a test has a high specificity?

Answer 9

non-diseased patient is more likely to test negative

Question 10

In what tests is it important to have high specificity? Why?

Answer 10

confirmatory tests

few non-diseased patients will incorrectly test positive (false positive)

high specificity rules in disease (SpIN) - confirms diagnosis

Question 11

SnOUT = _______
SpIN = _______

Answer 11

Sensitive test when Negative result rules OUT disease

Specific test when Positive result rules IN disease

Question 12

How should each lab have its reference intervals?

Answer 12

they should have their own RI for each species and their own lab location/environment

Question 13

How are reference intervals preferably established?

Answer 13

use 60-120 samples with the data analyzed to fit a normal (Gaussian) distribution - the typical values seen in 95% of healthy animals

• 2.5% of healthy animals will be below the RI
• 2.5% of healthy animals will be above the RI
• 1/20 healthy animals will have a result outside of the RI

Question 14

What is important to still note about results within the reference interval (WRI)?

Answer 14

may not necessarily be “normal”

• may not be normal for a given disease process
• common to have 2 disease processes “pushing” and “pulling” the result to be within the RI

Question 15

What are the most common pre-analytical error, analytical error, and post-analytical error?

Answer 15

PRE-ANALYTICAL - collection of sample
ANALYTICAL - testing the sample
POST-ANALYTICAL - human error in reporting results

Question 16

What are quality control (QC) programs?

Answer 16

involves analyzing quality control materials (QCM) that have pre-determined concentrations of an analyte, where each laboratory establishes it allowable error

Question 17

What indicates a test is highly precise?

Answer 17

same result with multiple runs or tightly clustered results

• can be precise, but inaccurate

Question 18

A 9 y/o MC Australian Shepherd presented to an emergency clinic after ingesting an entire bag of white chocolate Hershey’s kisses. Physical exam showed BCS 5/9, attitude was BAR, TPR 100.5 degrees F, 100 bpm, 32 breaths/min, and slightly pale MM with CRT < 2 sec.

However, venipuncture from the jugular vein was collected into an EDTA tube and run through an in-house hematology analyzer, showing a hematocrit of 0.11% (RI = 28-35%). Is it time to panic yet?

Answer 18

NO - physical and history are not consistent with the findings, as an animal with a 0.11% hematocrit would likely be dead

(PCV was not spun before being put in the hematology analyzer so it was not read correctly - reran and results showed a hematocrit of 22%)

Question 19

What are the most common indications of the following patterns?

• azotemia + urine specific gravity of unconcentrated urine
• azotemia + concentrated urine
• stress leukogram

Answer 19

• renal disease
• dehydration
• glucocorticoid (cortisone/prednisone)

pattern recognition is critical for making diagnoses