Intro to Diagnosis Flashcards
Why use diagnostics?
Make REASONED decisions about patient care based on clinical information and estimated outcomes
Used as a screening tool, to assist with diagnosis (urinalysis) and for patient management (measure blood glucose in diabetic)
Considerations
is it invasive? Is it expensive? Some diagnostics carry a risk of morbidity or mortality
False positives can lead to incorrect diagnosis or further unnecessary testing
Criteria used for screening tests:
Characteristic of population
characteristic of disease
characteristic of test
characteristics of population
sufficiently high prevalence of disease
likely to be compliant with subsequent tests and treatments
characteristics of disease
significant morbidity and mortality
effective and acceptable treatment readily available
pre-symptomatic period detectable
improved outcome from early treatment
characteristics of test!
good sensitivity and specificity, low cost and risk, confirmatory test available and practical
performance of diagnostic tests rely on what two things?
patient preparation (fasting, electrolyte restrictions, posture, physical activity prior, compliance)
specimen collection (proper labeling, test timing, medium of collection, proper site and technique [drawing blood above an IV], handling and storage)
test characteristics:
accuracy
precision
reference interval
interfering factors
sensitivity and specificity
sigma metrics (eh)
accuracy (bias)
test deemed inaccurate when result differs from true value, even if test is precise, AKA systematic error or bias
precision
if same specimen is analyzed many times, some variation (random error) is expected
this variability is expressed as a coefficient of variation percentage (CV)
ex: lab reports: serum creatinine with a CV of 5% and accepts results within 2 SD, so if expected result is 1, anywhere from 0.9-1.10 is accepted
sigma metrics
0-6
3 or less is bad
reference intervals in practice
(used on tests with quantitative results vs qualitative results)
represent test results found in 95% of a small pop presumed healthy, meaning 5% of healthy pop will have abnormal test result
Interfering factors (external and internal)
external: certain drugs/meds, contrast media, alcohol/cigs
internal: endogenous antibodies, diet, abnormal physiological states
sensitivity and specificity (both independent of prevalence of disease)
sensitivity: ability of test to detect disease expressed as % of patients with disease in whom the test was + (true positive vs false positive)
-commonly used for screening
-TP/TP + FN (TP/total diseased)
-vertical math
specificity: ability of a test to detect absence of disease expressed as % of patients without disease in whom test is - (true negative vs false negative)
-commonly used for definitive diagnoses
-TN/TN + FP (TN/total non-diseased)
-vertical math
positive predictive value
depends on prevalence of test in population
it is the probability of having the disease if test is positive
TP/TP + FP = PPV
negative predictive value
directly related to prevalence of disease
probably that a person is actually disease free if test is negative
TN/TN + FN = NPV
PPV vs NPV (horizontal math)
PPV: likelihood of having the disease when the test is positive
- directly related to prevalence
NPV: likelihood of not having the disease when the rest is negative
-inversely related to prevalence
prevalence equation
total positive/total number of patients x 100
methods of blood collection
venous, arterial, skin puncture
venous puncture
primary source of blood collection
most common is antecubital fossa of arm (mainly radial, sometimes ulna if Allen test permits)
basilic, cephalic and median cubital veins, femoral vein
venipuncture possible complications
bleeding, hematoma, infection, dizziness and fainting
arterial puncture
used to measure o2, co2 and pH, more difficult, more discomfort, brachial and radial most common used, may use femoral
skin puncture
Used in pediatric patients: finger tip (capillary), heel (in infants), ear lobes (very vascular)
preventing interfering factors
hemolysis: don’t shake tubes
Don’t collect from an arm with an IV running
avoid side with lymph node dissection
tourniquet shouldn’t be applied over a min.
basic lab tests:
blood chemistries
hematologic data
urinalysis
CSF
Serous body fluids (serous fluid fills body cavities)
urine methods of collection:
clean catch, mid-stream, catheter (best option for infection)
-contaminated specimen or true infection? no test is perfect
CSF collection
spinal tap
other body fluids
pleural (lungs)
peritoneal (abdomen)
synovial (joints)
amniotic
penile/vaginal
Blood chemistry tests
BMP: basic metabolic profile/panel
CMP: Complete metabolic profile/panel
-not always necessary! Ruzga would ask why
TEST: BMP includes what 8 tests?
Glucose, BUN, Creatinine, Sodium, Potassium, Chloride, Bicarbonate, Calcium (odd man out)
CMP includes:
previous 8, albumin, total protein, ALT, AST, ALKP, Bilirubin
focused also on liver function
-Albumin is a major binding protein made by the liver
-bilirubin is waste from red blood cells (that passes through liver before excretion)
CBC
complete blood count, helpful in evaluating common symptoms such as weakness, fatigue, fever, bruising
-diagnosing anemia, infection, leukemia, etc.
panel of tests: total WBC, differential, RBC count, hemoglobin concentration, hematocrit, platelet count, MCV, MCH, MCHC, and TDW
-hematocrit=ratio of RBCs to total blood volume
WBC
adult normal range: 4.5-11
panic: <0.5
determines total # as well as percentage and absolute # of each type of WBC
WBC cont.
Increased in acute (bacterial) infections
-tissue injury/necrosis (tissue death)
-allergies
-stress and smoking
decreased in prolonged (viral) infections (these infections can disrupt WBC production in bone marrow)
-alcoholism
-chemotherapy or radiation
TEST: Fives types of WBCs
Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils
RBC (erythrocytes) normal range
normal range:
males 4.3-6.0
females 3.5-5.5
Increases: hemoconcentration, dehydration
decreases: anemias, cold agglutination
TEST: HGB (hemoglobin) normal range:
Males 13.6-17.5 g/dL
females 12.0-15.5 g/dL
panic: <7.0 g/dL
increased in hemoconcentration (essentially water loss): dehydration, burns, vomiting
decreased in liver disease (heme made in liver), b12/iron/folate deficiency
TES: HCT (hematocrit) normal ranges
represents percentage of whole blood volume composed of RBCs
males 39-49%
females 35-45%
RULE of thumb: HCT is 3x the HGB value which is x3 the RBC value
MCV (mean corpuscular volume) normal range
It is the average volume of the RBCs themselves
used to determine if anemia is microcytic or macrocytic (above average RBC volume)
normal is 80-100 fL
Cleveland clinic: People may develop macrocytic anemia when they don’t get enough vitamin B12 and/or folate (vitamin B9) to create healthy red blood cells
MCH (Mean corpuscular hemoglobin) normal range
normal range: 27-33 pg
indicates the amount of HGB per RBC in absolute units (picograms, unlike MCHC which relates Hemoglobin content to cell volume)
MCHC (mean corpuscular hemoglobin concentration) normal values
normal range: 31-36 G/DL (relates hemoglobin to volume of cell often as %)
determine if anemia is hypochromic, normochromic or hyperchromic
average hemoglobin concentration in RBCs
Platelets normal range
released from megakaryocytes in bone marrow and important in hemostasis (repairing vessels)
normal range: 150-450,000/mcL
panic: <25,000
little tendency to bleed until platelet count <20,000
when to order platelets
suspected bleeding disorder, evaluating leukemia patients, DIC (clotting disease) monitoring chemo patients, etc.
Leukemia is a broad term for cancers of the blood cells. The type of leukemia depends on the type of blood cell that becomes cancer and whether it grows quickly or slowly. Cancer cells can crowd areas in the bone where platelets are made
TEST: PT (prothrombin time) test
evaluates the extrinsic and common coagulation pathways
-extrinsic = tissue damage, endothelial cells release factors
most sensitive to deficiencies in Vitamin K-dependent clotting factors: II, VII, IX and X
-sensitive to factor V
most commonly used for monitoring WARFARIN/COUMADIN therapy
-warfarin is anticoagulant
(not necessary test for preop unless clinically indicated)
PTT (partial thromboplastin time) normal range
normal 25-35 seconds
panic >60 seconds (unless using heparin)
evaluates intrinsic and common pathways and adequacy of all coagulation factors (except VII and XIII)
-intrinsic = factors in blood
commonly used to monitor HEPARIN therapy
-heparin is an anticoagulant
D-dimer overall!
One of the terminal fibrin degradation products
presence of d-dimers indicates that a fibrin clot was formed and subsequently degraded by plasmin (a proteolytic enzyme [breaks down proteins])
so, d-dimer is elevated whenever the coagulation system has been activated, followed by fibrinolysis
normal range: <400 ng/mL (<0.4)
increased in PE, DVT, VTE, etc. (situations needing clot dissolution)
D-dimer cont.
Sensitive test for DIC, DVT and VTE or PE
-exclude DVT/PE in patients with low or intermediate clinical probability
<400 in patients under 50 rules out PE/VTE but positive doesn’t confirm diagnosis
-age-adjusted cutoffs now being used to rule out PE
