interpretation of clinical lab tests Flashcards
if a test is highly sensitive, it means?
- test result is negative you can be nearly certain that the patient does not have the disease
- high rate of true negatives
highly sensitive tests are used to ______ for disease
screen
if a test is highly specific, it means?
- test result is positive you can be nearly certain that patient actually has disease
- high rate of true positives
highly specific tests are used to _____ for disease
confirm the results of sensitive
validity
measure of the test’s ability to indicate which individuals have the disease and which do not
reliability
- another term for consistency
- if test is admin repeatedly, does it yield the same results
incidence
- the number of new cases of a disease or condition in a specified time period divided by the size of the population under consideration who were initially disease free
incidence is a direct measure of?
disease risk - high incidence implies higher risk
prevalence
- actual number of cases alive with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence)
period prevalence vs point prevalence
- period prevalence provides better measure of the disease load since it includes all new cases and all deaths between 2 dates
- point prevalence only counts those alive on a particular date
low prevalence rate means
a lot of ppl die from the disease or the disease is cured fairly quickly
high prevalence rate means
a lot of ppl live with the disease
screening test is used to?
identify an asymptomatic individual who may have a particular disease
diagnostic test is sued to?
confirm the presence of a disease when a subject shows signs or symptoms of the disease
common screening tests
- pap smear
- fasting blood cholesterol
- fasting blood sugar
- BP
- fecal occult blood
- ocular pressure
- PKU test
- TSH
lab tests to workup for chest pain
- serum lactate dehydrogenase level
- serum creatinine phosphokinase level
- serum myoglobin levels
- serum troponin levels
lactate dehydrogenase
- an enzyme that catalyzes the conversion of lactate to pyruvate
- as cells die, their LDH is released and finds its way into the blood
lactate dehydrogenase levels in MI
- total LDH level rises within 24-48 hrs after an MI, peaks in 2-3 days, and returns to normal in 5-10 days
lactate dehydrogenase sensitivity and specificity for MI
diagnostic sensitivity and specificity is more than 90% within 24 hours of an MI when LDH-1 >40% of total; however even slight hemolysis can cause non-diagnostic elevations in LDH-1
creatinine phosphokinase
CPK enters blood rapidly following damage to mm cells but CPK levels rise and fall rapidly with a variety of other circumstances
sensitivity and specificity of CPK-MB for MI
- sensitivity and specificity of CPK-MB for MI are >90% within 7-18 hrs; peak concentrations occur within 24 hours and return to normal within 2-3 days
- sensitivity is poor when total CPK is very high
- specificity is poor when total CPK is low
myoglobin sensitivity for MI
not sensitivity enough to use for negative predictive value
troponin in dx MI
- criterion standard for dx MI
- higher specificity for MI than CK-MB
- higher long-term sensitivity than LD-1
troponin subunits seen in MI
cTnI and cTnT (troponins I and T)
lab test is useful in identifying the etiology of _____
anemia
CBC documents the ___ of the anemia
severity
what is shown on a CBC in chronic iron deficiency anemia?
mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) have values below normal range
when will CBC enter abnormal range during blood loss?
CBC will not enter abnormal range until most of the RBC produced before the bleed are destroyed at the end of their normal lifespan (120 days)
what will be seen on a peripheral smear in chronic iron deficiency anemia?
- microcytic and hypochromic RBC
- microcytosis is apparent in the smear before the MCV is decreased
how can MCV normalize?
- macrocytes mixed among the microcytic hypochromic cells
- occur in areas with folate and iron deficiency due to little fresh produce and meat
what is diagnostic of iron deficiency?
low serum iron and ferritin levels with an elevated TIBC
more sensitive laboratory indicators of mild iron deficiency
serum ferritin and stainable irone in tissue stores
how long before RBC indices become abnormal after tissue stores are depleted of iron?
several months
how is stool testing used in dx iron deficiency anemia?
hemoglobin in the stool is useful in establishing GI bleeding as etiology of iron deficiency anemia
how can bone marrow aspirate be diagnostic of iron deficiency?
absence of stainable iron in bone marrow aspirate permits establishment of a dx of iron deficiency w/o other lab tests
definition of oliguria
diminished capacity to form and pass urine -
UA findings in prerenal failure
- few hyaline and fine, granular casts
- little protein, heme, or red cells
- specific gravity of urine increased
UA findings in intrinsic renal failure
- hematuria and proteinuria are prominent
- broad, brown, granular casts found in ischemic or toxic acute tubular necrosis
- red cell casts found in glomerulonephritis
- white cells, especially eosinophils and white cell casts in acute interstitial nephritis
what is often used to distinguish decreased GFR from intrinsic renal disease?
fractional excretion of sodium
BUN/Cr in pre-renal and renal failure
pre-renal: >20/1
renal:
why is serum creatinine unreliable as indicator of renal damage?
- serum creatinine vary with age, lean mm mass, and hydration status
- serum levels may not change until >50% of kidney fx has been lost
- at lower GFR, increased tubular secretion of creatinine can lead to overestimation of renal fx
serum sodium is useful in dx ____?
cause of oliguria since urine output is influenced by serum sodium
hyponatremia usually indicates? hypernatremia usually indicates?
- hyponatremia is usually dilutional secondary to fluid retention or admin of hypotonic fluids
- hypernatremia is usually secondary to dehydration
how is serum potassium related to oliguria?
oliguria -> decreased GFR -> reduced tubular secretion -> metabolic acidosis -> increased serum potassium (decreased arterial pH raises serum potassium)
4 legitimate reasons for ordering a lab test
- dx
- monitoring
- screening
- research
approach to ordering lab tests
- shotgun - ordering a large number of lab tests that may or may not help in dx a particular disease
- rifle - ordering a specific lab tests based on how helpful it is in identifying a particular disease
