Question

In Community X (population 20,000), an epidemiologist conducted a prevalence survey in January of 2012 and reported an HIV prevalence of 2.2%. Over the next 12 months, the department of health reported an additional 50 new HIV cases between February 2012 and January 2013. The total population stayed constant at 20,000. Part 1 How many people had HIV in January 2012? Present or describe the formula you used to arrive at your answer. Calculate the incidence rate assuming no HIV-related deaths over the 12-month period. Present or describe the formula you used to arrive at your answer. Be sure to clearly indicate the numerator and denominator used in your calculation and include an appropriate label for the rate. In a summary of 200-250 words, interpret the results and discuss the relationship between incidence and prevalence. Discuss whether or not the epidemiologist should be concerned about these new HIV infections, assuming a previous incidence rate of 0.5 per 1,000 person-years prior to this updated risk assessment. Part 2 A rapid test used for diagnosing HIV has a sensitivity of 99.1% and a specificity of 90%. Based on the population prevalence of 2.2% in 2012, create a 2x2 table showing the number of true positives, false positives, false negatives, and true negatives. Calculate the positive predicative value and negative predictive value for this test

Answer 1

Population size in 2012 = 20,000

HIV prevelence in 2012 = 2.2%

PART 1:

a) Number of people with HIV in 2012 = Population size in 2012 * (HIV prevelence in 2012/100) = 20000*(2.2/100)

Number of people with HIV in 2012 = 440

b) Incidence rate = prevelence*1000 / (100*Time duration) = 2.2%*1000/ (100*1 year) = 1.83 per 1000 person-year.

Incidence rate = 1.83 per 1000 person-year

c) Since the incidence rat has been increased from 0.5 per 1000 person-years to 1.83 per 1000 person-years, the epidemiologists should be concerned.

PART 2:

a) True positive => TP; True negative => TN; False positive=>FP; False negative => FN

Number of Diseased (D) = 440

Number of non-diseased (ND) = Population size - D = 20000-440 = 19560

Sensitivity = TP / D => 99.1% = TP/440

TP = (99.1/100)*440 = 436.04

TP = 436

FN = D-TP =440-436 = 4

FN = 4

Specificity = TN/ND => 90% = TN/19560

TN = (90/100)*19560 = 17604

TN = 17604

FP = ND - TN = 19560/17604 = 1956

FP = 1956