Laboratory Medicine

T

T/F: laboratory tests are not usually diagnostic for a “specific disease”

F

T/F: laboratory tests can be ordered whether they could impact the management of the case or not

keep up with the state of knowledge & order tests for confirmation of dx

what is the Doctor’s legal obligation according to Classe?

1. review results & sign off

2. promptly & timely communicate results

3. be responsible for misinterpretation of results (by the lab or self)

what are things are within the professional legal liability of the OD to do when utilizing laboratory testing?

prevalence

proportion of individuals w/in a population that have a disease

positive predictive value

probability that a person w/ a positive test result actually has the disease

higher

the higher the prevalence, the ______ the PPV

increase

positive history and tentative dx ________ the PPV

F

T/F: every lab has the same standardized procedures/reference ranges

random screening

schedule of laboratory testing: FP & few true diseases

multiple testing

schedule of laboratory testing: odds of abnormal testing is high & Drs have a legal responsibility to order more testing if one is abnormal

false positive

positive lab result but no disease present; due to things like: cross rxns, biochemistry, poor specificity, random rxns

false negative

negative lab result but the disease is present; due to things like testing too early, biochemistry, poor sensitivity, poor sampling

sensitivity

measures how well a test identifies a positive result

specificity

measures how well a test identifies a negative result

specificity

a false positive result indicates the test could have poor _______(specificity/sensitivity)

sensitivity

a false negative result indicates the test could have poor _______(specificity/sensitivity)

when a lab value does not agree w/ the history & physical exam, the clinician should consider the lab in error

what is the rule of negative lab findings?

erythrocyte (RBC) count

measures the # of RBCs

higher

the RBC count is slightly ______ in males vs females

chemotherapy, vitamin B12 deficiency, hemorrhage, anemias

if a pt’s RBC count is low, what could this be secondary to?

relative polycythemia

an increase in Hgb, Hct, or RBC caused by a decrease in plasma volume

absolute polycythemia

too many RBCs

stress, smoking, dehydration, shock

what things can cause relative polycythemia?

altitude, COPD, hepatoma

what things can cause absolute polycythemia?

<2.0 & >7.0

what are the critical values for RBC count?

CVA secondary to hyperviscosity & thrombosis

if RBC count falls above/below the critical values, what are the risks?

hematocrit

• % of RBC to volume

• packed cell volume by centrifugation

• normal ranges vary by age/gender

MI

if Hct <20%, there is a risk of ___

CVA secondary to thrombosis

if Hct >60%, there is a risk of _______

<21% or >54%

what is the critical value for hematocrit?

Hemoglobin content

measure of how well cells are carrying oxygen around the body

higher

hemoglobin tends to be slightly _____ in males

<5.0 g/dL or >20.0 g/dL

what are the critical values for hemoglobin?

anemia, leukemia, lymphoma, over-hydration, severe burns, SLE, sarcoidosis

what can cause low hemoglobin?

COPD, CHF

what can cause high hemoglobin?

microcytic & hypochromic

if there is chronic blood loss, RBCs will be _______ (size & color)

normocytic & normochromic

if there is acute blood loss, RBC will be _______(size & color)

mean corpuscular volume (MCV)

measures the average volume/size of RBCs; best index for classifying anemias

mean corpuscular hemoglobin (MCH)

measures the average weight of Hgb in the RBC; calculated

false elevation

what effect does high cholesterol have on MCH?

microcytosis

small RBCs

macrocytosis

large RBCs

anisocytosis

variability in size of RBCs

red cell size distribution width (RDW)

• measures the degree of anisocytosis in RBC

• often done when RBC is low for differentially diagnosing

• high value in evaluating therapies

stained RBC film

• best means to evaluate size, abnormal shapes, & inclusions of RBCs

• good for things like: sickling, spherocytes, target cells, & fragments

platelet (thrombocyte) count

measure of # of platelets; clotting factors

<20,000 and >1million /uL

what are the critical values for platelet count?

thrombocytopenia

decreased # of platelets

thrombocytosis

increase # of platelets

septicemia, viral infections, most leukemias, autoimmune disease, menses, NSAID use, estrogen use

what might cause a low platelet count?

acute bacterial infection, acute bleeding, malignancy, iron deficiency anemia, surgery, pregnancy, inflammatory diseases

what things might cause an abnormally high platelet count?

high altitudes, cold climates, strenuous exercise

what things might case a normal high platelet count?

leukocyte (WBC) count

reflects the presence of infection or inflammation

leukemia, burns, bacterial infections, pregnancy, smoking, young age

what might cause a high leukocyte count?

viral infections, chemotherapy, NSAID usage, old age

what might cause a low leukocyte count?

leukocyte differential

• separate order w/ a CBC

• usually each type of leukocyte is reported as a relative percent of the total WBC

bands/stabs

term for immature neutrophils

neutrophils/segs

term for mature neutrophils

bacterial infections, RA, metabolic disease, pregnancy, stress

what might cause elevated neutrophils?

allergic disorders, parasites

what might cause elevated eosinophils?

leukemia, Hodgkin’s, neoplasms

what might cause elevated basophils?

viral infection, metabolic disease

what might cause elevated lymphocytes?

TB, SLE, RA, typhoid, endocarditis

what might cause elevated monocytes?

increased % of bands w/ an increase in segs, increase in total WBC

acute bacterial infection

describe a left shift & what is the most common cause?

increase in mature & hypersegmented leukocytes, drop in WBC

describe a right shift

there are abnormal hypersegmented leukocytes

what does a right shift imply?

anemia, liver disease

what are 2 common causes of a right shift?

neutropenia

_______ can look like a right shift; it is most often seen in children w/ viral infections

neutrophilia

increase neutrophil count

neutropenia

decreased neutrophil count

eosinophilia

increased eosinophil count

basophilia

increased basophil count

lymphocytosis

increased lymphocyte count

lymphopenia

decreased lymphocyte count

monocytosis

increased monocyte count

ESR, CRP, CPK/CK, cytokines: interleukin 2

what are the most often blood chemistries used in eye care?

ESR (erythrocyte sedimentation rate)

measure of how fast red blood cells settle out of a sample

faster

ESR: the ____ the erythrocytes settle out, the more settlement will be measured in the test tube in 1hr

fibrinogens

during inflammation, _______ will cause RBCs to stick together & fall faster

microcytic

________ RBCs fall faster & give a false positive result on ESR

westergren

what is the best ESR method?

age/2

how do you correct the standard ESR for men?

(age+10)/2

how do you correct the standard ESR for women?

c-reactive protein

• increases w/ inflammation, trauma, bacterial infection, surgery, neoplasms

• increases before antibody titers & ESR

• 100% sensitive for GCA but high FPs

MI risk

what is high-sensitivity C-reactive protein used for evaluating?

CPK/CK (creatine phosphokinase/creatine kinase)

• found in heart, skeletal muscles, & brain

• if elevated, indicates muscle damage

  • many other diseases can cause this

no

does a low CPK/CK mean anything?

T

T/F: the BB enzyme on CPK/CK testing is normal in MS & MG

T

T/F: CK-MB is good at detecting an MI w/in the 1st 6-24hrs

cytokines

secreted by WBCs in response to an immunologic challenge, includes interferons, interleukins, chemokines, inflammatory cytokines, & tumor necrosis factors

>600

high triglyceride levels (_____) invalidate the total cholesterol value on lipid determination tests

fasting

lipid determinations must be ordered _______

if pt is on oral diamox

when do we monitor electrolytes in eye care?

hypokalemia

low K+

hyperkalemia

high K+

increased

_______ levels on a renal function test suggest kidney disease

CT w/ contrast, concern for kidney clearance of an oral drug in elderly pts

when might we order renal function tests in eye care?

sarcoidosis testing

when might we order a serum lysozyme or ACE test in eye care?

steroids inhibit ACE activity

why might a pt on steroids for sarcoidosis get a false negative on ACE testing?