Lab Flashcards
Normal
Normal –
people in a non-diseased population; represented by data that fits into a bell-shaped curve
(Gaussian distribution)
Reference Range –
set of values of a measured quantity obtained from a group of individuals in a defined state of “health”
Prevalence
Incidence
Specificity
Sensitivity
Prevalence – number of cases of a disease in a certain population at a certain time
Incidence – number of new cases of a disease in a population during a specified time period
Specificity – probability that a lab test will be negative in absence of disease
Sensitivity – probability that a lab test will be positive in the presence of a disease
Predictive Values
Accuracy
Precision
Predictive Values – probability that a lab result accurately reflects the presence or absence of a disease
Accuracy – how closely the measurement approaches the true value
Precision – reproducibility
Mean
Median
Mode
Mean – the average
Median – divides the population in half; ranks the order of population from smallest to largest and the
middle value defines the median
o Point at which half are below and half are above (50th percentile)
Mode – peak, most probable value
Purpose of ordering tests
Detect/exclude possibility of disease
Confirm presence of disease
Classify type of disease
Monitor patient progress/response to therapy
Aid in choice of therapy
Specimen Collection and Handling
1) Pre-Analysis –
all the steps that take place before a specimen can be analyzed; major source of error
o Pre-Collection – must eliminate as many variables as possible:
Patient variables – fasting, diet, exercise, alcohol, smoking, drug use, age, posture
Things like diurnal variation (time of day), posture, and stress can affect tests a lot
o Ex: Cortisol, insulin, T4, & Calcium levels are all affected
Specimen Collection and Handling
2) Ordering the test –
can be electronic (the most error-free way) or written
o Include patient’s name, sex, age, DOB, actual date, clinic/hospital #, and physician’s name
o Computerized lab info systems – generate requisitions and specimen labels
o Provide number of tubes and type of tubes that are required
Consider the law – Medico-legal problems:
Proper patient ID
Proper specimen labeling
Proper patient consent
Privacy issues
Chain of custody
Specimen Collection and Handling
Proper patient ID
Proper specimen labeling
Proper patient consent
Privacy issues
Chain of custody
Specimen Collection and Handling
3) Collection
o Things to consider
Time of Collection – some specimens need to be collected at a specific time
Ex: cardiac markers, drug levels, glucose tolerance
Urgency – specimens labeled “STAT” or “ASAP” have priority in lab
The specimen
Must be adequate, representative of tissue, properly preserved, &correctly labeled
Types
o Excisional biopsy
o Incisional biopsy
o Needle biopsy
o Needle aspiration
o Cytologic smear
o Surgical Pathology – getting the specimen!
Communication w/ pathologist is key
-Notify a pathologist during a difficult case
-Gives patient’s history – previous lesions, lab studies, etc.
-Indicate the info you need from pathologist – margins, adequacy, etc.
Consider the clinical history
Has the patient had radiation/chemo and how long from when they’re biopsied?
During the procedure, describe…
- Appearance of tumor
-Orientation of specimen
-Location of biopsy sample
-Adequacy of biopsy sample
After the procedure – orient the specimen for margins
Specimen Collection and Handling
4) Preservation
4) Preservation
o Slide Prep – preparing the tissue w/ alcohol solutions and other fixatives
Problems
Fats are loss in the process (alcohol causes fat loss)
Heating (for paraffin blocks) causes artifacts (wrinkles, folding, etc)
Some tissues have to undergo special processing (ex: decalcification)
o Special Stains
Things that stain extracellular tissue components (collagen)
Mallory trichrome
Masson trichrome
Things that stains ECM fibers & reticulin fibers
Silver stains
Things that stain polysaccharides
PAS
o Frozen Section – ordered to help physician make a therapeutic decision
Relies on adequacy of specimen, margins, and nature of the lesion
5) Analyzing
o Cytology – used to screen for abnormalities or cancer; less invasive; diagnoses are made on individual cells or clumps of cells
o Immunohistochemistry – identifies cell products or surface markers by looking at availability
of specific antibodies
Used in diagnosis or management of malignancies
Categorization of tumors
Determination of the site of origin of metastasis
Detection of molecules w/ prognostic or therapeutic significance
o Flow Cytometry – rapidly and quantitatively measures several individual cell characteristics
Aids in the classification of certain tumors (t-cells, B-cells, monocytes/macrophages)
o Electron Microscopy – diagnosis the type of malignant tumor or works up disease in kidney, muscle, and nerve cells
o Molecular Diagnosis – useful in diagnosis and prognosis of malignant neoplasms, detection of minimal residual disease, diagnosis of hereditary predispositions, and choice of therapy
Distributions
Reference range
o To define the reference range, you have to define the population:
Based on age, gender, pregnancy, diet, tobacco use, exercise, etc.
o A bell-curve look (Gaussian)
o Ideally: mean, median, and mode are all equal
o Problems w/ reference range – using the upper and lower boundaries as rigidly defining
“normal” isn’t medically correct
Skewed Distributions –
non-Gaussian; either positive or negative
o Negative Skewness – higher mean than normal o Positive Skewness – lower mean than normal o Bimodal Distribution – 2 peaks
Decision Levels –
threshold values for lab test results
o When the value is beyond the decision levels, the clinical should
respond in some way o Examples
When serum calcium levels exceed 10.2mg/dL, you consider diagnosis of hyperparathyroidism and repeat the test
When serum calcium levels fall below 7mg/dL, you have to treat bc patient is at risk for developing tetany and convulsions
Diabetics take 1 unit of insulin for every 50mg/dL over 150mg/dL they are and eat if low
