Clinical Lab Testing Problems Highlights T. Davis

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

Specificity TN/TN+FP TN= 120 FP= 40 Specificity%= 120/ 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/ 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 ( 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= hrs: Myo= >1000; CK-MB= 0.9; TnI= 0.3

Case4 Myoglobin elevation due to skeletal muscle injury