clinical lab tests Flashcards
Sensitivity
If a person has a disease, how often will a test be positive?
If a test is highly sensitive, and it is negative, you can RULE OUT the disease.
(True positive)
BUT it can be positive for a lot of things
good predictive value, good for screening
high false positives
SNOUT= sensitive, rule out
ex: sed rate or ELISA for AIDS
Specificity
If a person has a disease, how often will the test be negative
if a test is highly specific, and it is positive, you can be certain they have the disease.
but if it is negative, you can’t rule that they do NOT have the disease
high rate of false negatives (borderline cases can be missed)
“SPIN”= specific, rule in”
used to confirm highly sensitive tests
ex: western blot
incidence
number of new cases in a specified time period
measure of rate (how often the disease appears)
implies disease risk
prevalence
the actual number cases exist at a given point in time #of cases/population
lactate dehydrogenase
enzyme that catalyzes conversion of lactate to pyruvate
LDH is released as cells die
rises within 24-48 hours after an MI, peaks in two to three days**=useful in pt with chest pain 36h ago
LDH1
heart-60%
RBC
kidney
sensitive and specific within 24h of an MI (>40%LDH)
high sensitivity
LDH2
heart-30%
RBC
kidney
LDH3
brain
kidney
LDH4
liver, muscle, brain, kidney
LDH5
liver
muscle
kidney
Creatinine Phosphokinase
CPK mostly resides in skeletal m, cardiac m. and brain
enters blood rapidly after damage to mm.
rises and falls quickly after surgery, vigorous exercise, IM injection
sensitivity poor when total CPK is high (good sensitivity when CPK is low, i.e. you can r/o MI when it is low)
specificity is poor with CPK is low (good specificity when CPK is high, i.e. r/in when it is high)
CK-1
BB
Brain
CK2
MB
Cardiac muscle
high S&S for MI w/in 7-18hrs (peak after 24h)
CK3
MM
Skeletal muscle
CPK relative index
measure of the mass of MB/Total
only used if total CPK is elevated
5= cardiac source
Myoglobin
O2 binding protein found in all tissues and cells, analog to Hb
peak level after mm damage at 6-12h
High specificity
not very sensitive, cant use for negative predictive value
Troponin
contractile protein not usually in serum
peak at 24-48h
criterion for standard dx by AHA
cTnI and cTnT are cardiac specific, and are usually very low, remain high for several days
specificity of CK-MB and the long term sensitivity of LDH-1
Iron deficiency anemia peripheral smears/CBC
microcytic and hypochromic
MCV and MCHC are low
with mixed in macrocytes
serum ferritin
low ferritin is diagnostic of iron deficiency: highly sensitive (normal ferritin can be present in people w/ low Fe), used to differentiate from chronic anemic disorders (RBCs don’t become abnormal until later)
normal can be seen in pt with Fe deficiency and hepatitis or chronic anemia
reticulocyte Hb content
with serum Fe, associated with iron deficiency anemia
Hb electrophoresis
identify HbA2, HbF, beta-thalassemia, or HbC/D as the etiology of microcytic anemia
Bone marrow aspiration
The absence of stainable iron in a bone marrow aspirate permits establishment of a diagnosis of iron deficiency without other laboratory tests.
No iron in bone marrow= dx of Fe deficiency (w/o other lab tests)
stool testing
presence of Hb is useful to establish the presence of GI bleeding as etiology for FeDA
prerenal failure and UA
fine granular casts with protein, heme, RBC
increased specific gravity
***fractional excretion of Na1020
Ratio of urinary to plasma creatinine is >40
Ratio of urinary to plasma osmolality is >1.5
Urinary sodium concentration is low <1%
intrinsic renal failure
hematuria and proteinuria
broad, brown granular casts with ischemic tubular necrosis
Ratio of urinary to plasma creatinine is 40 mEq/L
**Fractional excretion of sodium (FENa) >2%
BUN/Cre
pre-renal (dehydration): >20/1
renal: 50% of kidney function has been los
CBC and anemia
documents severity of anemia
Serum sodium
useful for dx of oliguria
hyponatremia=dilutional= fluid retention
hypernatremia= secondary to dehydration
Serum K
hyperkalemia is associated with decreased GFR
metabolic acidosis–> increases serum K
Ca and Pi
low GFR or kidney damage=
Hyperphosphatemia
Hypocalcemia
