Clinical Lab Testing Problems Highlights

T. Davis 9-2-2014

Problem 1 Statistics

• New test for gastric CA• 180/200 pts with gastric CA have a (+)

test• 40/160 pts w/o gastric CA have a (+) test• In your clinical practice 1/200 pts have

gastric CA• Calculate: sensitivity, specificity, PV+• How do you improve PV+?

What is sensitivity?

• The percent of patient with gastric CA that have a positive test; a highly sensitive test misses few pts with disease

• What is a true positive?• What is the formula for sensitivity?

True Positive (TP) = gastric CA plus a (+) test

• Sensitivity% = TP/TP+FN x100• FN= gastric CA but a (-) test

• 180/180+20= 90%

How about specificity? How often does it call a (-) a (+)?

• TN- pts without gastric CA and a (-) test• FP- pts without gastric CA and a (+) test

SpecificityTN/TN+FP

• TN= 120• FP= 40

• Specificity%= 120/120+40 x100• Specificity= 75%

Predictive Value**(population-sensitive)

• What % of (+) tests are really positive• What % of (-) tests are really negative

• PV(+)% = TP/TP+FP x100• PV(-)% = TN/TN+FN x100

Prevalence and Incidence• Prevalence- # of people

in the population with disease currently

• Cummulative (total)

• Diabetes has high prevalence and low incidence

• Incidence- annual # of people who get disease

• Annual (new cases)

• Common cold has high incidence and low prevalence

Prevalence:# pts in the study with the

disease/total population studied

• TP+FN/TP+FP+TN+FN

• In this population it is 1/200• If you see 10,000 pts and run the test, 9,950

do not have gastric CA and 50 have gastric CA

PPV continued

• TP= 90% x 50= 45• FN= 10% x 50= 5• TN= 75% x 9,950= 7,462• FP= 25% x 9,950= 2,488

• PV+%= TP/TP+FP x100• = 45/45+2,488 x 100• = 1.8%

If the prevalence is 10% what is the PPV+?

• 1000 of 10,000 have the disease• 1000/1000+2488 x 100• About 28%

Prevalence and Tests

• A higher prevalence increases the PV+• A lower prevalence decreases the PV+

Case 1• 68-y.o.male with wt loss, anorexia, nausea and constipation.

Mucous membranes and sclera are icteric. A 5 cm-mass is palpated in the RUQ.

• Bili (tot)= 7.1 mg/dL (<1.0)• Bili(d)= 3.4 mg/dL (<0.2)• AST= 102 U/L (<40U/L)• ALT= 88 U/L (<40 U/L)• AlkPhos= 506 U/L (115 U/L)• gammaGT= 258 U/L (35 U/L)

Case1 (cont.)

• d/tot Bili= >4• Transaminases are slightly elevated• Alk phos and gamma GT are markedly

elevated, suggesting obstruction• Pt had CA of the head of the pancreas

Case 2• 6 y.o. male with fever, anorexia, vomiting and recent

onset of abdominal pain and lassitude. Sclera were icteric and the spleen was palpable.

• Bili (tot)= 6.8 mg/dL• Bili (d)= 0.6 mg/dL• Plasma-Free Hb= 26 mg/dL (<10 mg/dL)• Haptoglobin= 0 mg/dL (35-200 mg/dL)

Case 2 (cont.)

• Hemolysis suspected- d/tot bili< .2• Plasma HB is elevated and haptoglobin is

decreased, both c/w hemolysis

• Pt had hereditary eliptocytosis

• Why was the spleen palpable?

How would a case of hepatitis present?

• d/totBili?• Transaminases?• Alk Phos?• Plasma free-Hb and Haptoglobin?

Case 3

• 53-y.o. male presents to the ER (ED) with chest pain for 16 hours.

• EKG c/w MI• Myo 358 (<110ng/mL)• CK-MB 12.9 (<5 ng/mL)• TnI 32.8 (<0.5ng/mL)

Case 3 is an acute MI

• Myoglobin positive at 2 hrs post infarct; it offers the least specificity

• TnI positive at 4 hrs (3-6) post infarct• CK-MB positive at 6 hrs (3-6) post infarct

• Would this patient be a candidate for clot busters or angioplasty?

Case 4

• 75-y.o. male presented to the ER (ED) after MVA with chest pain. EKG is nondiagnostic. MI vs contusion

• 6 hrs: Myo= >1000; CK-MB= 2.1; TnI= 0.3• 16 hrs: Myo= >1000; CK-MB= 0.9; TnI= 0.3

Case4

• Myoglobin elevation due to skeletal muscle injury