Laboratory Medicine :

Introduction and General

Principles

Laboratory Medicine Conference Series :Goals and Objectives

ƒ Review principles of decision analysis as they apply to lab tests

ƒ Review background physiology and measurement methodology for lab tests

ƒ Present sensitivity, specificity, & general clinical usefulness of specific lab tests

ƒ Determine cost effectiveness of common lab tests

Laboratory Medicine Conference Series :Lecture Topics Listƒ 1. Introduction & Decision Analysisƒ 2. Leucocyte, Erythrocyte, Platelet, & Clotting

Studiesƒ 3. Electrolytes, Calcium, Glucose, & Osmolality ƒ 4. Renal & Liver Function Testsƒ 5. Urinalysisƒ 6. Cardiac Enzymes & Markersƒ 7. Amylase, Lipase, Pregnancy Tests, Sedimentation

Rateƒ 8. Arterial Blood Gasesƒ 9. Sputum & Pleural Effusion Testsƒ 10. Cerebrospinal Fluid Analysisƒ 11.Toxicology Studiesƒ 12. Miscellaneous Microbiology & Serology Studies

Why Discuss the Rather Unexciting Topic of Laboratory Medicine ?

ƒ Knowledge of test methodology helps in analysis of test limitations

ƒ Many lab tests ordered actually have no impact on clinical decision making

ƒ Potential to save large amounts of money by more appropriate test ordering

ƒ Stimulate ideas for research projects to assess lab tests' clinical usefulness & accuracy

Further Reasons that Discussion of Lab Medicine is Important

ƒ 1/4 to 1/3 of current lab testing is estimated as unnecessary

ƒ Overordering contributes to the false impression that the E.D. is "too expensive"

ƒ Practicing cost-effective medicine represents good medical practice

ƒ Clinically unimportant abnormal lab values can "label" patients & cause them unnecessary anxiety

Problems with Inappropriate or Unnecessary Ordering of Lab Tests

ƒ Wastes moneyƒ May delay definitive careƒ Extra discomfort for patient if

blood draw otherwise unnecessary

ƒ Extra risk to patient of investigating or treating false positive results

ƒ False assurance about disease presence if test result is unrecognized false negative

What are Some of the Reasons Behind the Current Ordering of Too Many Lab Tests ?

ƒ "Habit is easier than thinking"ƒ Anticipating expectations of

supervising physicians ; wanting to appear complete & compulsive

ƒ False concept that this provides legal protection

Sequential Steps in Evaluation of an Individual Lab Test

ƒ Analytic analysis–Precision, accuracy, technical sensitivity & specificity of the test procedure

ƒ Diagnostic analysis–Relation of test result to presence or absence of disease

ƒ Operational analysis–Predictive clinical value of test result

ƒ Cost - benefit analysis–Cost, safety, & relation of test to remainder of clinical decision tree

Reasons to Order Laboratory Tests

ƒ Good reasons–Screening–Diagnosis–Management –Research

ƒ Crummy reasons–Idle curiosity–Placebo effect–Medicolegal concerns–Peer review concerns

Lab Tests Ordered for Screeningƒ Screening = test not predicated on signs or

symptoms of diseaseƒ Requirements for a good screening test :–Disease in question should be common enough in the patient population to justify efforts to detect it–Disease should have significant morbidity if undetected–Effective intervention for the disease is possible–The test should be abnormal in a very high % of patients with the disease

Four General Processes of Diagnostic Reasoning

ƒ Formation of hypothesesƒ Exclusion of some hypotheses

under considerationƒ Inclusion of some hypotheses

for further considerationƒ Use of these hypotheses to

guide the search for further information

Definitions of Terms Referring to Lab Tests

ƒ Accuracy–Degree of agreement between test result & known standard value

ƒ Precision–Ability of particular lab testing method to produce same exact results time after time

ƒ Sensitivity–Probability that test is abnormal in a patient with the disease

ƒ Specificity–Probability that test result is normal or negative in the absence of disease

Test Predictive Value Matrix

+ - + A B

- C D

Presence of Disease

Lab Test Results

Sensitivity = A / (A + C)Specificity = D / (B + D)Positive Predictive Value = PPV = A / (A +B)Negative Predictive Value = NPV = D / (D + C)Prevalence = (A + C) / (A + B + C + D)

Definitions of Test Interpretation Termsƒ Positive Predictive Value (PPV) :

–Estimate of % of time that a patient will actually have the disease if the test is positive–Or, the % of patients with a positive test who have the disease

ƒ Negative Predictive Value (NPV) :–Estimate of % of time that the patient will be free of disease if the test is negative–Or, the % of patients with a negative test result who do not have the disease

Definitions of Test Interpretation Terms (cont.)

ƒ Prevalence–Probability that a patient in the group under study already has the given disease before the time of the study–Or, the % of all individuals under study who have the disease

ƒ As the prevalence of the disease increases, the PPV increases & the NPV decreases

Definitions of Test Interpretations

Test Value Disease present No Disease

Abnormal True Positives (TP) False Positive (FP)

Normal False Negatives (FN) True Negative (TN)

Sensitivity (SS) TP / TP + FN TP / all with disease

Specificity (SP) TN / TN + FP TN/ all without disease

Predictive Value of Positive Test (PPV)

TP / TP + FP TP / all with positive test

Predictive Value of Negative Test (NPV)

TN / TN + FN TN /all with negative test

Interpretations of PPV and NPV

ƒ As prevalence increases, value of a positive test becomes more meaningful

ƒ At low prevalence, value of a positive test (even if its sensitivity & specificity are high) is low

ƒ Negative test result for disease of low prevalence virtually excludes disease (if sensitivity & specificity are high)

ƒ Test with high PPV can be useful to "rule in" a disease

ƒ Test with high NPV most useful to "rule out" a disease

Interpretation of Test Predictions

ƒ A test can have 100 % sensitivity & specificity only if the distribution of test results is nonoverlapping between diseased & non-diseased patients

ƒ Prevalence of most diseases in unselected populations is < 5 %

ƒ So use of tests to screen for disease in asymptomatic or low incidence population ends up that most positive results are false positive (false negative rate also high)

Example of Effect of Prevalence on PPV : *Prostatic acid phosphatase as predictor of prostate cancer ( sensitivity 70 %, specificity 90 % ) :

Setting Prevalence (cases / 100,000)

PPV (%)

General population 35 0.4

Men age 75 or more

500 5.6

Prostate nodule present

50,000 93.0

* N.E.J.M. 1980 ; 303: 497-499.

Probability That a Healthy Person Will Have Abnormal Results in a Biochemical Profile *

Number of Tests Probability (%) That at Least One Test Will Be Abnormal

1 5

6 26

12 46

20 64

100 99.4

*Ann Int Med 1987 ; 106 : 403-413

Benefit to Cost Ratio Analysis

ƒ Threshold probability (pT) = probability of disease at which there is no difference in outcome between treatment or Dx and lack of Rx or Dx

ƒ So when benefit to cost ratio is low, test needs a high pT (high PPV) to be of use

ƒ When the benefit to cost ratio is high, the degree of uncertainty can be greater

ƒ When benefit to cost ratio is low, test with high NPV not required to exclude a Dx

ƒ When trying to exclude a Dx, & the benefit to cost ratio is high, a test with a high NPV is needed

"Point of Service" (Bedside) Lab Testing

ƒ Results available much quickerƒ No need for specimen transportƒ Less blood drawnƒ At present is 10 to 20 times

more expensive than "in-lab" tests

ƒ Can prevent use of lab tests as a delaying or holding technique

"Clinical Laboratories Improvement Act “ (CLIA)

ƒ Federal law passed to regulate all office & hospital labs

ƒ Requires :–Specific staff training & prerequisites–lab Q / A program–Periodic inspections–Lots of paperwork & fees to register each lab

ƒ Regulations apply even to stool guiacs, urine dipsticks, fingerstick glucoses, etc.

A.C.E.P. Lab Test Cost Containment Project

ƒ Estimated that up to 30 % of E.D. lab tests are unnecessary

ƒ Estimated that proper use of guidelines could save $ 5.9 billion annually (this # of $ would provide health insurance for 1.6 million people)

ƒ Overuse of even low charge items adds up to a lot of $ in aggregate

Frequently Ordered Lab Tests for Medicare Patients in 1987 (HCFA data)

Laboratory Test Allowed Services Allowed Charges ($) Routine ECG 26,116,333 534,714,028

Routine venipuncture

20,227,087 60,652,091 CBC 16,276,672 150,480,379 CXR 14,641,479 318,617,533 Urinalysis 13,659,116 73,651,635 SMA-12 7,751,248 135,974,256 Glucose 7,638,169 49,762,334 Prothrombin time 3,667,127 24,963,037 Thyroxine level 2,812,774 28,176,841 Potassium 2,629,100 18,465,803 High density lipoprotein

2,353,452 28,469,571

Examples of Overuse of Lab Tests in Emergency Medicine

ƒ "Routine" trauma or admission labs not specific to the patient's illness

ƒ CBC to "uncover" bacterial illness

ƒ PT / PTT prior to anticoagulationƒ Electrolytes in simple

gastroenteritis or brief period of vomiting or diarrhea

Preop Labs Required at H.M.C.

ƒ Age 1 day to 44 years : hematocrit

ƒ Age 45 to 59 years :–Hct, glucose, creatinine, ECG

ƒ Age > 60 : above tests plus CXRƒ Add other labs as indicated

Review of Preoperative Lab Screening (J.A.M.A. 1985; 253 : 3576)

ƒ 2000 patients had CBC, lytes, glucose, PT / PTT preop

ƒ only 40 % of these tests were indicated

ƒ There were only 10 unsuspected abnormal results :–6 were ignored–one was incorrectly noted as "normal"–Clinical impact of all the tests was zero

Sample Study on Lab Usefulness :CBC for Bacterial Disease (Ann Emer Med 1986 ; 15 : 774)

ƒ Retrospective of 172 E.D. patientsƒ 21 % sent by R.N. & not used by

M.D.ƒ PPV (WBC > 12.5) only 26 %ƒ PPV of left shift only 33 %ƒ Cost $ 10,100 per clinical impact

Sample Study of Lab Test Usefulness :PTT Prior to Anticoagulation (Ann Emer Med 1993 ; 22 : 697)

ƒ Reviewed 199 E.D. admissions with DVT or Pulm. embolus

ƒ Only 3 abnormals not predicted by history (one congenital, 2 on warfarin)

ƒ Heparin was always started prior to return of lab results

ƒ Caused no changes in heparin therapy

Sample Study of Lab Test Usefulness :Electrolytes for Simple Seizure (Ann Emer Med 1986 ; 15 :33)

ƒ 180 E.D. patients with seizureƒ 27 (17 %) of cases had low

bicarbƒ Other lytes were zero to 7 %

abnormal–Did not affect clinical management

ƒ One unsuspected hypoglycemia

Labs Recommended for New Onset Seizure by Different References

ƒ Harrison :–Na, Ca, Glucose, TFT's, Lead, Arsenic

ƒ Rosen :–CBC, U/A, Glucose, Lytes, Ca, ABG

ƒ A.C.E.P. Clinical Policy for Seizure :–No labs–Consider CT or MRI

Study of Criteria Predictive of Clinically Significant Electrolyte Abnormalities (CSEA)(Lowe et al., Ann Emer Med 1991 ; 20 : 16)

ƒ Criteria Used :–Poor oral intake, vomiting, chronic HBP, diuretic use, recent seizure, muscle weakness, age > 65, alcoholism, abnormal mental status, recent electrolyte abnormality

ƒ Prospectively validated these criteria in 984 E.D. patients

ƒ Sensitivity was 94 %ƒ Eight "misses" :

–4 with low bicarb or Cl ; 2 had results circled in chart but not acted on ; 2 had low K but empirically replaced

ƒ Following the criteria would have led to 24 % reduction in obtaining lytes with no adverse clinical outcomes

Study of Clinical Guidelines Used to Reduce Lab Testing (Gen Int Med 1990 ; 5 : 335)

ƒ Involved 79 M.D.'s & 1638 hospital patients

ƒ Used guidelines from consensus panel

ƒ Voluntary participationƒ Resulted in 21 % reduction in lab

tests orderedƒ No change in clinical outcomes as

measured by death or readmission within 90 days

Principles of Serologic Methods

ƒ Four general classifications of serologic tests :–Agglutination–Precipitation–Complement fixation–Fluorescent antibody

ƒ All 4 utilize measurement of reaction of antigen & antibody

Principles of Agglutination Tests

ƒ Can be done in test tube or on slide–Slide proceure : mix patient's serum with antigen on slide & observe for agglutination–Test tube procedure : serial dilutions of serum mixed with constant amount of antigen in successive tubes ; the highest dilution showing agglutination is the antibody titer

Principles of Indirect Hemagglutination Tests

ƒ One type uses viral or parasitic antigens that are adsorbed to erythrocytes of animals (chickens, rabbits, sheep or horses) or humans

ƒ Another type uses antigens absorbed onto latex particles–Allows longer storage & use of slide tests

Principles of Precipitation (Precipitin) Tests

ƒ Show precipitation of antigen-antibody complex

ƒ Can be done with capillary tubes where precipitin forms at interface of antigen & antibody

ƒ Or can be done with agar wells which allow antigen & antibody to diffuse into each other to form a line of precipitation–If electric current applied (counterimmunoelectrophoresis), this increases the speed of diffusion

Principles of Complement Fixation Tests

ƒ Mix patient's serum with antigen & complement, & then mix in red blood cells coated with anti-RBC antibody

ƒ If any unbound complement is then available, it will bind to the RBC's & lyse them

ƒ If there is no lysis, then all the complement has been bound to the original complex & the presence of antibody in the patient's serum is indicated

Principles of Fluorescent Antibody Tests

ƒ Uses a fluorescent dye (such as fluorescein isothyocyanate) which binds to antibody

ƒ The antigen-antibody complex is then directly observed with a fluorescent microscope

What Can You Do for More Appropriate Lab Test Ordering ?

ƒ Only order tests that you would be willing to pay for or have done on yourself

ƒ Use clinical criteria for ordering when these are available

ƒ Know charges for common testsƒ Educate your colleaguesƒ Help develop clinical guidelines

Principles of Lab Test Ordering : Summary

ƒ Perform test if additional information required for diagnosis or management

ƒ Don't perform test if result will not alter management

ƒ Select test based on need to rule in or out particular disease

ƒ Review test methodology, sensitivity & specificity, & cost - benefit ratio

ƒ Assess test result in terms of PPV or NPV