clinical lab tests

Question 1

Sensitivity

Answer 1

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

Question 2

Specificity

Answer 2

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

Question 3

incidence

Answer 3

number of new cases in a specified time period
measure of rate (how often the disease appears)

implies disease risk

Question 4

prevalence

Answer 4

the actual number cases exist at a given point in time
#of cases/population

Question 5

lactate dehydrogenase

Answer 5

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

Question 6

LDH1

Answer 6

heart-60%
RBC
kidney

sensitive and specific within 24h of an MI (>40%LDH)

high sensitivity

Question 7

LDH2

Answer 7

heart-30%
RBC
kidney

Question 8

LDH3

Answer 8

brain

kidney

Question 9

LDH4

Answer 9

liver, muscle, brain, kidney

Question 10

LDH5

Answer 10

liver
muscle
kidney

Question 11

Creatinine Phosphokinase

Answer 11

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)

Question 12

CK-1

Answer 12

BB

Brain

Question 13

CK2

Answer 13

MB
Cardiac muscle
high S&S for MI w/in 7-18hrs (peak after 24h)

Question 14

CK3

Answer 14

MM

Skeletal muscle

Question 15

CPK relative index

Answer 15

measure of the mass of MB/Total
only used if total CPK is elevated
5= cardiac source

Question 16

Myoglobin

Answer 16

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

Question 17

Troponin

Answer 17

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

Question 18

Iron deficiency anemia peripheral smears/CBC

Answer 18

microcytic and hypochromic
MCV and MCHC are low

with mixed in macrocytes

Question 19

serum ferritin

Answer 19

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

Question 20

reticulocyte Hb content

Answer 20

with serum Fe, associated with iron deficiency anemia

Question 21

Hb electrophoresis

Answer 21

identify HbA2, HbF, beta-thalassemia, or HbC/D as the etiology of microcytic anemia

Question 22

Bone marrow aspiration

Answer 22

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)

Question 23

stool testing

Answer 23

presence of Hb is useful to establish the presence of GI bleeding as etiology for FeDA

Question 24

prerenal failure and UA

Answer 24

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%

Question 25

intrinsic renal failure

Answer 25

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%

Question 26

BUN/Cre

Answer 26

pre-renal (dehydration): >20/1

renal: 50% of kidney function has been los

Question 27

CBC and anemia

Answer 27

documents severity of anemia

Question 28

Serum sodium

Answer 28

useful for dx of oliguria

hyponatremia=dilutional= fluid retention

hypernatremia= secondary to dehydration

Question 29

Serum K

Answer 29

hyperkalemia is associated with decreased GFR

metabolic acidosis–> increases serum K

Question 30

Ca and Pi

Answer 30

low GFR or kidney damage=
Hyperphosphatemia
Hypocalcemia