Diagnosis by Lab Methods Flashcards
sensitivity
rate of true positive of test
very sensitive - always positive for someone who has disease
good negative predictive value
if positive - can’t conclude they have disease
specificity
good positive predictive vale
positive specific test - 100% certain have disease
negative - doesn’t mean they don’t have disease
tests with high specificity
used to CONFIRM results of sensitive, but less specific screening test
incidence
number of new cases of disease in specified period
divided by size of population under consideration who were initially disease free
measure of RATE
prevalence
actual number of cases during period of time or at particular date in time
measure of COMMONALITY
chronic incurable disease
low incidence but high prevalence
highly contagious, rapidly fatal disease
high incidence, but low prevalence
work up in chest pain labs
- serum lactate DH
- serum creatinine phosphokinase
- serum myoglobin
- serum troponin
1975
galen first use if CPK, LD, and isoenzymes
1985
CK MB
1991-1992
troponins
lactate DH
LDH
-enzyme catalyzes coversion of lactate to pyruvate
cells die - release LDH to blood
seen in many different cells
not good predictive value
good thing about LDH
stays high in serum for long time
useful in someone with bad chest pain 36 hours ago
-may still be elevated
hemolysis also causes elevation of LDH-1
-NOT THAT SPECIFIC
CPK
catalyzes reversible transfer of P between creatinine and phosphocreatine as well as between ATP and ADP
BB - brain
MB - cardiac
MM - skeletal m
sensivitiy/specificity of CPK-MB
> 90%
within 7-18 hours
quicker and more specific
CPK relative index
mass of CPK-MB fraction / total CPK x 100
ratio less than 3 - skeletal m source
ration grater than 5 - cardiac source
myoglobin
O2 binding in muscle tissue
sensitive test - but not enough
levels rise early after muscle damage
elevated 2-4 hours after symptoms, peak 6-12 hours, normal 24-36 hours
temporal changes in CK-MB and myoglobin
myoglobin rapid increase
CK - MB - less rapid increase
troponin
regulatory protein in striated muscle
calcium channels open - Ca causes troponin to change shape allowing actin and myosin to bind leading to contraction
three subunits
-TnC - binds Ca
TnT - binds tropomyosin
TnI - binds actin
troponin levels
gold standard for determining myocardium damage
serum troponin levels
increase 3-12 hours after onset of chest pain, peak 24-48 hours, return to baseline 5-14 days
criterion standard*** for diagnosing myocardial infarction
troponin C
same all muscle tissue
troponin I and T
cardiac specific
-these ones used
78yo F - weak and dizzy, HTN, lethargic, mucous membrane pale, murmur, lungs CTA
anemic
work up for anemia
blood loss - most common
marrow - decreased production
hemolysis - destruction
iron deficiency - most common etiology
most common cause of anemia
blood loss
lab tests for anemia
RBC count peripheral smear serum iron total iron binding serum ferritin retic count RBC distribution
elevated bilirubin
hemolysis
CBC
give you indices
- MCV
- MCHC
- chronic iron deficiency anemia - both below normal range
after blood loss - cellular indices do not enter abnormal range until after most erythrocytes destroyed (120 days)
anemia
ranked basis of size and Hg content
microcytic hyperchromic
-and others
**know this
microcytic hypochromic erythrocytes on peripheral smear
chronic iron deficiency anemia
folate and iron deficiency
common in areas of world with little fresh meat and produce
peripheral smear - population of macrocytes** mixed among microcytic hypochromic cells
can normalize the MCV
sickle cell anemia
see on peripheral smear
not rounded and concave
- RBCs misshaped
- looked like sickles
doesn’t hold O2
Dx of iron deficiency
- low serum iron and ferritin
- elevated TIBC
normal serum ferritin - deficienct in iron with coexistent diseases
hepatitis or anemia of chronic disorders
red blood cell indices
do not become abnormal for several moths after tissue stores are depleted of iron
hemoglobin studies
Hg A2 and fetal Hg
- electrophoresis - measure of Hg A2 and fetal Hg
- useful for establishing beta-thalassemia or Hg C or D as etiology of microcytic anemia
reticulocyte Hg content
and serum iron are only parameters independently associated with iron deficient anemia
stool testing
Hg in stool - GI bleed
very common etiology for anemia
absence of stainable iron in bone marrow aspirate**
permits establishment of diagnosis of iron deficiency without other lab tests
61yo M - ICU, repair of ruptured aneurysm, lost blood, transfused, urine output has been slowing
oliguria
oliguria
diminished capacity to form and pass urine less than 500mL in 24 hours
imbalance of body fluids / electrolytes
renal disease
urinary tract obstruction
prerenal failure
hyaline and fine, granular casts observed on urinalysis
increased specific gravity
renal failure
hematuria and proteinuria are prominent
-brown granular casts are typically found in ischemic or toxic acute tubular necrosis
acute interstitial nephritis
white cells in urinalysis
eosinos and white cell casts
postrenal
obstruction
prerenal
high specific gravity
high urine:plasma Cr
low urine sodium
low FENa
hypovolemia**
renal
urine:plasma Cr
osmolality ratio 40
FENa >2
intrinsic renal disease**
BUN/Cr
> 20:1 prerenal
<20:1 renal failure
Cr - varies with age
lose 50% of kidney function - may not have change in serum Cr
serum Na
useful in diagnosis cause of oliguria
hyponatremia
fluid retention, administration of hypotonic fluid
hypernatremia
secondary to dehydration
serum K
oliguria - decreased GFR - reduced tubular secretion - metabolic acidosis - increased serum potassium
0.1 reduction in arterial pH
raises serum potassium by 0.3
reasons for ordering lab test
diagnosis**
monitor
screen
research
shotgun-ordering
large number of lab tests that may or may not have adequate diagnostic predictive value in identifying particular disease
rifle-ordering**
specific lab tests based on assessment of their diagnostic accuracy and predictive value in identifying a particular disease
how will test results influence patient management?
important question before ordering a lab test
