A study is conducted to assess the effectiveness of a new medication X added to standard antihypertensive therapy compared to standard antihypertensive therapy alone for preventing the development of proteinuria in patients recently diagnosed with essential hypertension. Five years after diagnosis, 97 of 100 patients on the medication X + standard antihypertensive therapy regimen have not developed proteinuria, and 90 of 100 patients on a standard antihypertensive therapy regimen have not developed proteinuria. According to these results, which of the following represents the approximate number of patients with essential hypertension who need to be treated with medication X to prevent an additional patient from developing proteinuria within 5 years?
Common measures of therapeutic efficacy | ||
Term | Definition | Calculation |
Absolute risk reduction (ARR) | Percentage indicating the actual difference in event rate between control & treatment groups | ARR = control rate – treatment rate |
Relative risk reduction (RRR) | Percentage indicating relative reduction in the treatment event rate compared to the control group | RRR = ARR / control rate |
Relative risk (RR) | Ratio of the probability of an event occurring in the treatment group compared to the control group | RR = treatment rate / control rate |
Number needed to treat (NNT) | Number of individuals who need to be treated to prevent a negative outcome in 1 patient | NNT = 1 / ARR |
The number needed to treat (NNT) is the number of patients who need to be treated with a specific drug (eg, medication X) to prevent an additional negative event (eg, 1 fewer patients with essential hypertension who develop proteinuria within 5 years of diagnosis) when compared with a control group (eg, standard antihypertensive therapy). NNT is the inverse of the absolute risk reduction (ARR):
NNT = 1 / ARR
The ARR describes the difference in risk of an unfavorable event (eg, development of proteinuria within 5 years of hypertension diagnosis) between the treatment group (eg, medication X + standard antihypertensive therapy) and the control group (eg, standard antihypertensive therapy):
ARR = (Riskcontrol − Risktreatment)
In this question, 100 − 90 = 10 out of 100 patients in the control group developed proteinuria within 5 years of hypertension diagnosis, so Riskcontrol is 10/100 = 0.10. Similarly, 100 − 97 = 3 out of 100 patients in the treatment group developed proteinuria within 5 years of hypertension diagnosis, so Risktreatment is 3/100 = 0.03. Therefore, the ARR for development of proteinuria within 5 years of hypertension diagnosis in the treatment group is:
ARR = (Riskcontrol − Risktreatment) = (0.10 − 0.03) = 0.07
and the NNT is:
NNT = 1 / ARR = 1 / 0.07 = 14.3 ≈ 15
NNTs are always rounded up to the nearest whole number, so approximately 15 patients need to be treated with medication X + standard therapy to prevent 1 additional case of proteinuria within 5 years of hypertension diagnosis.
(Choices A and C) 3 and 10 represent the number of patients in the treatment group and in the control group, respectively, who developed proteinuria within 5 years of diagnosis.
(Choice B) 7 represents the ARR expressed as a percentage, calculated as: ARR = 100 × (Riskcontrol − Risktreatment) = 100(0.10 − 0.03) = 7.
(Choice E) 70 represents the relative risk reduction (RRR) expressed as a percentage. The RRR describes the relative decrease in the risk of an adverse event in the treatment group compared to a control group; it is calculated as: RRR = 100 × [(Riskcontrol − Risktreatment) / Riskcontrol] = 100[(0.10 − 0.03) / 0.10] = 70.
Educational objective:
The number needed to treat (NNT) is the number of patients who need to be treated with a specific treatment to avoid an additional negative event. NNT is the inverse of the absolute risk reduction.