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What percentage of diagnoses rely to some extent on a lab result?



What are some of the general reasons to order a lab test?

diagnose a disease
screen for disease
risk assessment of future disease
monitor disease progression
monitor response to treatment


What are the general principles of lab diagnosis?

Generally speaking, labs look for changes from normal in regards to:
1. Damage to cells (leakage of itnracellular contents)
2. Failure of excretory processes from organ damage
3. Increased amount of tissue-of-origin (tumor markers)
4. Co-incident patterns of biochemical markers
5. Genetic factors rapidly emerging


In general, how is the reference range made?

population mean plus or minus 2 std deviations

so includes 95% of observations in the healthy population


Based on how the reference range is made, 5% of values will be outside the range even in a healthy person. If n independent tests are performed on a healthy person, what number are predicted to be abnormal?

1 - (0.95)^n

if you run a test 20 times, you have a 64% likelihood of finding at least one abnormal value


In some cases we use a more stringest criterion to establish the abnormal level. What's the main example of this?

troponin needs to be over the 99th percentile of the upper reference limit of a normal population to define AMI


How was the cholesterol range defined?

by a committee (national cholesterol education program)

in fact, when you look at the actual range for cholesterol, the mean is 201 (above "normal" and about 20% of the population is above 240!)


What is the diagnostic definition of a test's sensitivity?


How many people with the disease test positive?


What is the diagnostic definition of a test's specificity?


How many people without the disease will test negative


Usually normal and diseased values overlap, so if you increase sensitivity, you decreased specificity and vice versa. What is the graphical representation of this called?

a receiver-operator curve

plot of sensitivity vs. 1-specificity


What do you look for on a receiver-operator curve to indicate the ability fo a test to separate the groups of interest (healthy vs. diseased)?

You measure the area under the ROC curve

higher values will indicate better ability to separate the groups


What is the definition of positive predictive value?

If someone tests positive for a disease, how likely is he/she to have the disease?



What is the definition of negative predictive value?

If someone tests negative for disease, how likely is he/she actually healthy



True or false: disease prevalence affects tests sensitivity and specificity.

false - it affects the predictive values, not the sensitivity/specificity

higher prevalence = fewer FP, so higher positive predictive value

lower prevalence = more false positives, so lower predictive value


Screening in low prevalence groups is therefore inherently limited. For example, what is the PPV of the minnesota CAH (newborn screen)

only 8%!!


What are the general areas where error can be introduced to lab testing?

test selection/ordering
specimen collection
specimen identification/labeling
transport to lab
clerical issues
result reporting to ordering physician


What is the most common type of error?

wrong specimen labeling


What is the info the lab needs to therefore make a test accurate?

correctly label the sample with name, time, other identifiers

requisition form completed with correct tests checked

biopsy site, orientaiton, clinical impression

your contact info including info for calling emergency values


(check course website for list of critical values at immediate risk to the patient)



What is the difference between "routine" and "stat"?

stat samples get priority in the queue, so the routine ones may need to wait

ONLY use stat orders if really medically urgent