Stats >> Training >> Stats #24: Practice Exercises

1. Read the following abstract. The likelihood ratios for a PSA cutpoint of 4 ng/ml were deleted. Calculate these values using the remaining information in the abstract. Interpret these values.

Prostate-specific antigen testing accuracy in community practice. Hoffman RM, Gilliland FD, Adams-Cameron M, Hunt WC, Key CR. BMC Fam Pract 2002: 3(1); 19. [Medline] [Abstract] [Full text] [PDF]

BACKGROUND:
Most data on prostate-specific antigen (PSA) testing come from urologic cohorts comprised of volunteers for screening programs. We evaluated the diagnostic accuracy of PSA testing for detecting prostate cancer in community practice.
METHODS:
PSA testing results were compared with a reference standard of prostate biopsy. Subjects were 2,620 men 40 years and older undergoing (PSA) testing and biopsy from 1/1/95 through 12/31/98 in the Albuquerque, New Mexico metropolitan area. Diagnostic measures included the area under the receiver-operating characteristic curve, sensitivity, specificity, and likelihood ratios.
RESULTS:
Cancer was detected in 930 subjects (35%). The area under the ROC curve was 0.67 and the PSA cutpoint of 4 ng/ml had a sensitivity of 86% and a specificity of 33%. The likelihood ratio for a positive test (LR+) was [DELETED] and [DELETED] for a negative test (LR-). PSA testing was most sensitive (90%) but least specific (27%) in older men. Age-specific reference ranges improved specificity in older men (49%) but decreased sensitivity (70%), with an LR+ of 1.38. Lowering the PSA cutpoint to 2 ng/ml resulted in a sensitivity of 95%, a specificity of 20%, and an LR+ of 1.19.
CONCLUSIONS:
PSA testing had fair discriminating power for detecting prostate cancer in community practice. The PSA cutpoint of 4 ng/ml was sensitive but relatively non-specific and associated likelihood ratios only moderately revised probabilities for cancer. Using age-specific reference ranges and a PSA cutpoint below 4 ng/ml improved test specificity and sensitivity, respectively, but did not improve the overall accuracy of PSA testing.

2. Read the following abstract. The likelihood ratios for H. pylori culture were deleted. Calculate these values using the remaining information in the abstract. Interpret these values and the associated confidence intervals..

Laboratory diagnosis and susceptibility profile of Helicobacter pylori infection in the Philippines. Destura RV, Labio ED, Barrett LJ, Alcantara CS, Gloria VI, Daez ML, Guerrant RL. Ann Clin Microbiol Antimicrob 2004: 3(1); 25. [Medline] [Abstract] [Full text] [PDF]

BACKGROUND:
Helicobacter pylori diagnosis and susceptibility profile directs the applicability of recommended treatment regimens in our setting. To our knowledge, there is no published data on the culture and local susceptibility pattern of Helicobacter pylori in the Philippines.
METHODS:
52 dyspeptic adult patients undergoing endoscopy from the Outpatient Gastroenterology clinic of the University of the Philippines-Philippine General Hospital underwent multiple gastric biopsy and specimens were submitted for gram stain, culture, antimicrobial sensitivity testing, rapid urease test and histology. Antimicrobial susceptibility testing was done by Epsilometer testing (Etest) method against metronidazole, clarithromycin, amoxicillin, and tetracycline.
RESULTS:
Sixty percent (60%) of the study population was positive for H. pylori infection (mean age of 44 years +/- 13), 70% were males. H. pylori culture showed a sensitivity of 45% (95% CI [29.5-62.1]), specificity of 98% (95%CI [81.5-100%]), positive likelihood ratio of [DELETED] (95% CI [1.254-317.04]) and a  negative likelihood ratio of [DELETED] (95% CI [0.406-0.772]). All H. pylori strains isolated were sensitive to metronidazole, clarithromycin, amoxicillin and tetracycline.
CONCLUSION:
Knowledge of the antibiotic susceptibility patterns in our setting allows us to be more cautious in the choice of first-line agents. Information on antibiotic susceptibility profile plays an important role in empiric antibiotic treatment and management of refractive cases.