1 Absolute Risk Reduction, Number Needed to Treat, Back-of-the-Envelope Cost Effectiveness Analysis, Treatment Thresholds Revisited 1 November 2012 Michael.

Absolute Risk Reduction, Number Needed to Treat, Back-of-the-Envelope Cost Effectiveness Analysis, Treatment Thresholds Revisited

1 November 2012Michael A. Kohn, MD, MPP

Using Randomized Trials to Quantify Treatment Effects

Diagnosis: Evaluate a test and then use it to determine whether a patient has a given disease. (Chs. 2, 3, 4, 5, 8)

Treatment: Determine if a treatment is beneficial in patients with a given disease, and if so, whether the benefits outweigh the costs and risks. (Chs. 9, 10)

In screening programs (Ch. 6), diagnosis and treatment are the most closely intertwined. Prognostic testing (Ch. 7) requires longitudinal studies and evaluation of calibration as well as discrimination.

EBM is about using research studies to help in two related areas

Quantifying the Benefit of a Treatment: Take Home Points

RCT Checklist Need baseline incidence of bad

outcome*. Number Needed to Treat =NNT= 1/ARR Number Needed to (treat to) Harm =

NNH = 1/ARI Back-of-the-envelope CEA: Treatment

cost per bad outcome prevented = Treatment Cost x NNT

*Unless the RR is 1 and RRR is 0.

RCT Checklist

Design and conduct Randomization to address issues of confounding Blinding of patients and clinicians to prevent

differential co-interventions Blinding of outcome assessors to prevent bias Patient-Oriented Effect Measures (POEMs) vs.

surrogate outcomes Decompose composite outcomes Good follow-up to eliminate differential losses to

follow-up

*For checklist on study validity, see Chapter 1B1 “Therapy”, in Guyatt and Rennie (eds.), Users Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice; AMA Press; 2002.

RCT Checklist for Study Validity*

Analysis Intention-to-treat analysis (once

randomized always analyzed) Compare entire randomization groups, not

subgroups Between groups rather than within groups

comparison

*For checklist on study validity, see Chapter 1B1 “Therapy”, in Guyatt and Rennie (eds.), Users Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice; AMA Press; 2002.

RCT Checklist for Study Validity*

Randomization, Intention-to-Treat Analysis, and Follow-up: Hip Replacement vs. Screws

Pt is an 81-year-old woman with a hip fracture Pt’s son is a physician. He asks about hip

replacement vs. screws.

Pubmed search Parker MJ, Khan RJ, Crawford J, Pryor GA. Hemiarthroplasty versus internal fixation for displaced intracapsular hip fractures in the elderly. A randomised trial of 455 patients. J Bone Joint Surg Br. Nov 2002;84(8):1150-1155.

Displaced Femoral Neck Fracture = Hip Fracture

9Internal Fixation = Screws

10Hemiarthroplasty = Hip Replacement

Randomization, Intention-to-Treat Analysis, and Follow-up: Hip Replacement vs. Screws

Randomized controlled trial of the effects of hip replacement vs. screws on re-operation and other outcomes in > 70-year-old patients with displaced, hip fractures.

Parker MH et al. Bone Joint Surg Br. 84(8):1150-1155.

Randomization: Hip Replacement vs. Screws

Why do a randomized experiment?Why not do an observational study

comparing mortality, re-operation rates, etc. between patients who had hip replacements and patients who had screws?

Intention-to-Treat: Hip Replacement vs. Screws

Some patients randomized to the hip replacement group ended up getting screws.

Why not include these patients’ outcomes in the screws group or at least exclude them from the hip replacement group?

How to analyze those who were assigned to hip replacement but received screws?*

Intention to Treat: Analyze in the hip replacement group (reduces effect size, bias towards the null)

Per Protocol: Exclude from analysis (bias in favor of hip replacement)

As Treated: Analyze in the screws group (bias in favor of hip replacment)

*Generally they are older, weaker, or sicker than the rest of the group.

Losses to Follow-Up: Hip Replacement vs. Screws.*

If each treatment group had 20% loss to follow-up, there could still be bias.

What if those in the screws group were lost to follow-up because they got better and those in the hip replacement group were lost because they died?

*In fact, there were no losses to follow-up in this study.

Patient Oriented Endpoints, Blinding: Arthroscopy versus Immobilization for 1st Shoulder Dislocation

Pt is a 34-year-old man who dislocated his shoulder while surfing at Ocean Beach.

He asks about early arthroscopic stabilization versus immobilization and PT.

Pubmed search Kirkley A, Griffin S, Richards C, Miniaci A, Mohtadi N. Prospective randomized clinical trial comparing the effectiveness of immediate arthroscopic stabilization versus immobilization and rehabilitation in first traumatic anterior dislocations of the shoulder. Arthroscopy. Jul-Aug 1999;15(5):507-514

Outcomes Affected by Treatments*

Dichotomous (e.g., recurrent dislocation)

Continuous (e.g., Western Ontario Shoulder Disability Index {WOSI})

Endpoints Patient relevant (e.g., ability to return to

sports) Surrogate (e.g., MRI findings)* Example: Arthroscopy vs. conservative tx for 1st Anterior Shoulder Dislocation (Arthroscopy. 1999 Jul-Aug;15(5):507-14. )

Outcomes Affected by Treatments

Dichotomous (e.g. recurrent dislocation)

Continuous (e.g. WOSI)Endpoints Patient relevant (e.g., ability to

return to sports) Surrogate (e.g., MRI findings)

Blinding

Blinding of Patients and Clinicians Eliminates differential co-

interventions

Blinding of Outcome Assessment Eliminates biased outcome

assessment (including placebo effect)

BlindingBlinding less important when opportunity for

cointerventions that affect outcomes is minimal, and outcome is not subjective.

Hip Replacement vs Screws for hip fracture, with endpoints of mortality and re-operation: patients, clinicians, and outcome assessors not blinded.

Arthroscopy vs. non-operative management of shoulder dislocation, with endpoints of re-dislocation, and WOSI*: patients not blinded, but clinicians and outcome assessors (therapists) were blinded.

*Western Ontario Shoulder Disability Index

Between-groups Comparison: ApoA-I Milano vs. Placebo to Reduce Atheroma Volume in Acute Coronary Syndrome

Nissen SE, Tsunoda T, Tuzcu EM, Schoenhagen P, Cooper CJ, Yasin M, et al. Effect of recombinant ApoA-I Milano on coronary atherosclerosis in patients with acute coronary syndromes: a randomized controlled trial. Jama 2003;290(17):2292-2300.

Percent Atheroma Volume in the Target Coronary Segment

N Baseline Follow-Up Change P Value*

Placebo 11 34.8 34.9 0.1 0.97

ApoA-I Milano

15 mg/kg 21 39.7 38.4 -1.3 0.03

45 mg/kg 15 37.9 37.2 -0.7 0.45

Combined 36 39.0 37.9 -1.1 0.02

* P values for within-group comparison from Wilcoxon signed rank test. For between-group comparison, P = 0.29

Between-groups Comparison: ApoA-I Milano vs. Placebo to Reduce Atheroma Volume in Acute Coronary Syndrome

Sub-group Analysis: ISIS II*30-day mortality

*Lancet 1988;2(8607):349-360.

Overall Geminis and

Libras

Other signs

Aspirin 9.4% 11.1% 9.0%

Placebo

11.8% 10.2% 12.1%

Sub-group Analysis: ISIS II*30-day mortality

*Lancet 1988;2(8607):349-360.

Aspirin Placebo

Died Total % Died Total %

Gemini/Libra 150 1359 11.0% 147 1442 10.2%

All other signs 654 7228 9.0% 868 7157 12.1%

Composite Endpoints: Irradiation to prevent re-blockage after cleaning out a blocked coronary artery bypass graft.

Major Cardiac Event

Yes No

Bypass

Irradiation 19 41 60 32%

Placebo 38 22 60 63%“At 12 months, … the rate of major cardiac events was 49 percent lower (32 percent vs. 63 percent, P<0.001). “

Waksman, R., A. E. Ajani, et al. (2002). "Intravascular gamma radiation for in-stent restenosis in saphenous-vein bypass grafts." N Engl J Med 346(16): 1194-9.

“Major Cardiac Event” = Death or MI or Revascularization

Procedure

Yes No

Bypass Graft

Irradiation 4 56 60 6.7%

Placebo 4 56 60 6.7%

DEATH or MI

Yes No

Bypass Graft

Irradiation 5? 54 60 8%

Placebo 6? 55 60 10%

Can’t tell from paper.

DEATH or MI

Yes No

Bypass Graft

Placebo 0 60 60 0%

Could have been…

Mortality or MI worse but composite endpoint better.

*For checklist on study validity, see Chapter 1B1 “Therapy”, in Guyatt and Rennie (eds.), Users Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice; AMA Press; 2002. (Or try http://www.cche.net/usersguides/therapy.asp#Valid )

DONE: RCT Checklist for Study Validity*• Randomization to address issues of confounding • Blinding of patients and clinicians to prevent differential

co-interventions• Blinding of outcome assessors to prevent bias• Patient-Oriented Effect Measures (POEMs) vs.

surrogate outcomes• Take care with composite outcomes• Good follow-up to eliminate differential losses to follow-

up• Intention-to-treat analysis (once randomized always

analyzed)• Between groups rather than within groups comparison• Compare entire randomization groups, not subgroups

1. Hip replacement versus screws for hip fractures in the elderly.

2. Immediate arthroscopy versus immobilization and PT in first shoulder dislocation.

3. ApoA-I Milano vs. placebo in acute coronary syndrome.

4. Aspirin versus placebo in suspected acute myocardial infarction: ISIS-2.

5. Intravascular gamma radiation vs. placebo to prevent re-blockage after cleaning out a blocked coronary artery bypass graft.

Effect Size(Dichotomous Outcomes*)

RRRRRARRNNTARINNH

* Not going to discuss continuous outcomes today

This study was properly randomized but not blinded, used an intention-to-treat analysis, and had NO losses to follow-up.

Results follow…

Hip Replacement vs. Screws

Reduced Re-operation

Re-operation No Re-operation Risk

Hip Replacement 12 217 229 12/229 = 5.2%

Internal Fixation with Screws 90 136 226 90/226 = 39.8%

Risk Ratio (RR): 5.2%/39.8% = 0.13

Relative Risk Reduction (RRR): 1 - RR = 87%

Absolute Risk Reduction (ARR):39.8% - 5.2%

= 34.6%

Number Needed to Treat (NNT) 1/ARR = 3

(Need to treat 3 patients with hip replacement instead of screws to prevent one patient requiring re-operation.)

Measures of Treatment Effect

RR= Risk Ratio =

RR < 1 means treatment is beneficial

RRR = Relative Risk Reduction = 1-RR

Bad Outcome

No Bad Outcome

Totals

Treatment a b a + b

Control c d c + d

Totals a + c b + d N = a + b + c + d

Beware of the Odds Ratio

RR = Risk Ratio =

(a/b) (a/c)

OR = Odds Ratio = ------- = -------- = ad/bc

(c/d) (b/d)

Bad Outcome

No Bad Outcome

Totals

Treatment a b a + b

Control c d c + d

In the hip replacement vs. screws example, the baseline risk of reoperation (with screws) is 40%,

so the baseline odds are 67%. The risk (or odds) with replacement is about 5% , so RR ≈ 5/40 ≈

1/8; but the OR ≈ 5/67 ≈ 1/13.

Major Cardiac Event

Yes No

Bypass Graft

Placebo 38 22 60 63%

Irradiation to prevent re-blockage after cleaning out a blocked coronary artery bypass graft.

“The risk of a major cardiac event was significantly lower in the iridium-192 group than in the placebo group (odds ratio, 0.27; 95 percent confidence interval, 0.13 to 0.57; P<0.001) “

Major Cardiac Event

Yes No

Bypass Graft

Placebo 38 22 60 63%

RR = (19/60)/(38/60) = 0.50OR = (19/41)/(38/22) = 0.27

Measures of Treatment Effect

ARR = Absolute Risk Reduction = c/(c+d) - a/(a+b)

NNT = Number Needed to Treat (to prevent 1 bad outcome) = 1/ARR

Bad Outcome

No Bad Outcome

Totals

Treatment a b a + b

Control c d c + d

41Q: What does the 34% reduction mean?

Nimotop® Ad Graph

22% 33%

RR = 21.8%/33% = .66 RRR = 1-0.66 = 34% ARR = 33% - 21.8% = 11.2%

Original figure

To scale

Why is NNT = 1/ARR?

67 no stroke anyway

22 strokes with Nimotop®

11 strokes prevented

22 strokes with with treatment

33 strokes with no treatment

100 SAH patients treated

Why is NNT 1/ARR?

Treat 100 SAH patients; prevent 11 strokes.

100/11 = 1/11% = 1/ARR = 9 patients treated per stroke prevented.

Number Needed to Treat …

With what? To prevent what? In whom?

NNT PracticeIn patients < 30 years old with first-time acute

anterior shoulder dislocation, prompt arthroscopic surgery (vs. standard conservative therapy) reduces the 2-year re-dislocation rate by almost 33% in absolute terms (from about 50% to about 17%).*

How many first-time dislocation patients do we need to treat with arthroscopy to prevent one having re-dislocation at 2 years?

*Kirkley A, et al. Arthroscopy. Jul-Aug 1999;15(5):507-514. Numbers rounded for purposes of exposition.

NNT Practice

ISIS- 2*. Aspirin therapy (one month of 160 mg/day) in patients with acute myocardial infarction (AMI) reduced 30-day cardiovascular mortality from 11.8% in the placebo group to 9.3% in the aspirin group.

*Lancet 1988;2(8607):349-360.

NNT Practice

How many AMI patients do we need to treat with aspirin to prevent one CV death at 30 days?

Death at 30 Days

Died Lived

Aspirin 804 7783 8587 9.3%

Placebo 1016 7584 8600 11.8%

*Lancet 1988;2(8607):349-360.

The risk ratio (RR) or relative risk reduction (RRR = 1-RR) associated with a treatment is of minimal use without knowing the baseline level of risk*.

Problem with the Relative Risk Reduction

*The RR is not completely useless without the baseline risk. If RR=1, the tx is useless regardless of the baseline risk. If RR << 1, then the treatment is beneficial; if RR >> 1, the treatment is harmful. Also, if you already know the baseline risk in your own population, the RR may be all you need.

“At 12 months, the rate of revascularization of the target lesion was 70 percent lower in the iridium-192 group than in the placebo group”

After cleaning out blocked bypass graft, how many do we have to treat with iridium to prevent one revascularization procedure (of the target lesion)?

“At 12 months, the rate of revascularization of the target lesion was 70 percent lower in the iridium-192 group than in the placebo group”

After cleaning out blocked bypass graft, how many do we have to treat with iridium to prevent 1 revascularization procedure in the next 12 months?

The baseline risk (i.e., risk in the placebo group) was 57%

Baseline 12-month risk of revascularization = 57%

RRR = 0.70ARR = RRR × Riskcontrol

ARR = 0.70 × 0.57 = 0.40NNT = 1/0.40 = 2.5Need to treat 2.5 unclogged grafts with radiation

to prevent 1 from needing revascularization of the target lesion in the next 12 months.

Problem with the Relative Risk Reduction: Need Baseline Risk

ARR = RRR × Riskcontrol

Analysis and Presentation of Results

Relative risk most relevant for assessing causation, more easily generalized

Absolute risk reduction (ARR) most relevant for clinical decisions

1/(ARR) = Number Needed to Treat (NNT) per outcome prevented

Flu Prophylaxis? Pt is a 6-year-old girl with fever, myalgias, cough

and sore throat X 1 day Should you rx prophylactic Tamiflu® for the pt’s

mother who is pregnant?

Pubmed search Welliver R et al. Effectiveness of Oseltamivir in Preventing Influenza in Household Contacts: A Randomized Controlled Trial. JAMA 2001; 285:748-754.

Prophylactic Oseltamivir: Index Case Flu+*

Household Contacts

Flu No Flu Risk

Oseltamivir 3 206 209 3/209 = 1.4%

Placebo 26 180 206 26/206 = 12.6%

29 386 415

Risk Ratio (RR): 1.4%/12.6% = 0.11

Relative Risk Reduction (RRR): 1 - RR = 89%

Absolute Risk Reduction (ARR): 12.6% - 1.4% = 11.2%

Number Needed to Treat (NNT) ????

*Welliver R et al. Effectiveness of Oseltamivir in Preventing Influenza in Household Contacts: A Randomized Controlled Trial. JAMA 2001; 285:748-754.

Prophylactic Oseltamivir: Index Case Flu+*

Household Contacts

Flu No Flu Risk

Oseltamivir 3 206 209 3/209 = 1.4%

Placebo 26 180 206 26/206 = 12.6%

29 386 415

Risk Ratio (RR): 1.4%/12.6% = 0.11

Relative Risk Reduction (RRR): 1 - RR = 0.89

Absolute Risk Reduction (ARR): 12.6% - 1.4% = 11.2%

Number Needed to Treat (NNT) 1/ARR = 9

*Welliver R et al. Effectiveness of Oseltamivir in Preventing Influenza in Household Contacts: A Randomized Controlled Trial. JAMA 2001; 285:748-754.

Number Needed To HarmNausea No Nausea Risk

Oseltamivir 27 467 494 27/494 = 5.5%

Placebo 12 449 461 12/461 = 2.6%

Risk Ratio (RR): 5.5%/2.6% = 2.1

Absolute Risk Increase (ARI): 5.5% - 2.6% = 2.9%

Number Needed to Harm (NNH): 1/ARI = 35

NNH is really number needed to treat to cause one undesired effect.

Number Needed to Harm

Not an apt term for number needed to treat to cause one bad outcome.

Would prefer NNTc (“Number Needed to Treat to cause”) vs. NNTp (“Number Needed to Treat to prevent”), but NNH is well established.

Number Needed to HarmTransfusion No Transfusion

Hip Replacement 44 179 223 44/223 = 19.7%

Risk Ratio (RR): 19.7%/1.8% = 11.00

Number Needed to Harm (NNH) ????

Number Needed to HarmTransfusion No Transfusion

Hip Replacement 44 179 223 44/223 = 19.7%

Risk Ratio (RR): 19.7%/1.8% = 11.00

Number Needed to Harm (NNH) 1/ARI = 6

(Need to treat 6 patients with hip replacement instead of screws to cause one patient requiring transfusion.)

Ratio of Undesired to Desired Effects

“Harms” / Bad Outcome Prevented =ARI/ARR =NNT/NNH OrBad Outcomes Prevented / Harm Caused

= ARR/ARI =NNH/NNT

Ratio of Desired to Undesired Effects

Harms Caused / Bad Outcome Prevented = ARI/ARR = NNT/NNH*Hip Replacement vs. Screws for Hip FxRisk Difference for re-operation:

∆ Risk Re-Op = 5.2% - 39.8% = -34.6%

Risk Difference for transfusion: ∆ Risk Trx = 19.7% - 1.8% = +17.9%

Transfusion Caused/Reoperation Prevented: 17.9/-34.6 = -0.52 ≈ -1/2

*Easier here to divide ∆ transfusion by ∆ re-operation, rather than use NNH or NNT.

Ratio of Desired to Undesired Effects

Bad Outcomes Prevented / Harm Caused = ARR/ARI = NNH/NNT*Hip Replacement vs. Screws for Hip FxRisk Difference for re-operation:

∆ Risk Re-Op = 5.2% - 39.8% = -34.6%

Risk Difference for transfusion: ∆ Risk Trx = 19.7% - 1.8% = +17.9%

Re-operations prevented/Transfusion Caused: -34.6/17.9 = -1.93 ≈ -2

*Easier here to divide ∆ re-operation by ∆ transfusion, rather than use NNH or NNT.

Ratio of Undesired to Desired Effects

Cases of Nausea / Flu Case Prevented =

2.9%/ 11% = 0.25 OrFlu Cases Prevented / Nausea

Caused = 11%/2.9% = 4

BOTE CEA

Back-of-the-Envelope Cost Effectiveness Analysis

(In book, this is referred to a “treatment cost per bad outcome prevented.)

Back-of-the-Envelope Cost Effectiveness Analysis

How many patients do I need to treat (at the treatment cost) to prevent 1 bad outcome?

Number Needed to Treat (NNT) = 1/ARR

Cost of preventing one bad outcome =NNT x Treatment Cost*

*This is just ∆$Cost /∆Risk .

BOTE CEA: Oseltamivir

Index Case Flu + NNT = 9 (Treat 9 household contacts,

prevent 1 flu case) NNT x Treatment Cost* = 9 x $50 =

$450/flu case prevented

•Cost of Tamiflu 75 mg #10 = $92.99 www.drugstore.com 11/4/09

BOTE CEA: Aspirin after MI

• NNT = 1/0.025 = 40 (Treat 40 MI patients to prevent 1 death at 30 days)

• At Rite Aid, a bottle (#120) of 81 mg aspirin tablets costs $5.00, but you only need 60.

• NNT x Treatment Cost* = 40 x $2.50 = $100/death prevented

Death at 30 Days

Died Lived

Aspirin 804 7783 8587 9.3%

Placebo 1016 7584 8600 11.8%

*Lancet 1988;2(8607):349-360.

BOTE CEA Example

Letrozole (Femara®) to prevent breast cancer recurrence after 5 years of tamoxifen therapy.

Drug cuts risk of breast-cancer relapse

Findings so promising, study halted so scientists could release news

By Sabin RussellChronicle Medical Writer

Front Page, San Francisco Chronicle 10/10/03

RCT of Letrozole (Femara®), after tamoxifen, to prevent breast cancer recurrence

RRR or ARR?

“The trial was interrupted almost 2½ years after it began. Researchers had scheduled a midpoint peak at the data, and found letrozole was apparently working far better than expected. The women who took it had 43 percent fewer recurrences of their breast cancer compared to those assigned in the study to take a placebo, or dummy pill.”

Femara Trial Results

RecurrenceNo

Recurrence

Letrozole 75 2500 2575

Placebo 132 2450 2582

Risk(Letrozole) = 75/2575 = 2.9%Risk(Placebo) = 132/2582 = 5.1%RR = 2.9/5.1 = 0.57RRR = 1- 0.57 = 43%

N Engl J Med. 2003 Nov 6;349(19):1793-802.

Femara Trial Results

ARR = 5.1% - 2.9% = 2.2%NNT = 1/2.2% = 45Treatment Cost = $266/month* x 12

months/year x 2.5 years = $7980Femara Cost per Recurrence Prevented =

$7980 x 45 ≈ $360,000

*2.5mg tablets are available from www.drugstore.com $266/30 day supply (30 tablets) 1/7/2008.

BOTE CEA Examples Oseltamivir to prevent flu in household

contacts of flu+ individuals:$450 per case of flu prevented

Aspirin after acute MI: $100 per death prevented at 30 days

Letrozole after tamoxifen to prevent recurrent breast cancer: $360,000 per recurrence prevented at 2.5 years

BOTE vs. “Real” CEA

Estimates treatment costs per bad outcome prevented – including the bad outcome’s costs

Treatment Costs---------------------------------------------------Bad Outcome + Bad Outcome’s Costs

“Real” Cost Effectiveness Analysis:

Treatment Costs – Bad Outcome’s Costs-------------------------------------------------------

Bad Outcome

BOTE vs. “Real” CEABack of the envelope:

Treatment Costs---------------------------------------------------Bad Outcome + Bad Outcome’s Costs

“Real”:

Treatment Costs – Bad Outcome’s Costs-------------------------------------------------------

Bad Outcome

Note that “real” analysis LOWERS the cost per bad outcome prevented and makes treatment look better.

Quantifying the Benefit* of a Treatment: Take Home Points The Risk Ratio or Relative Risk Reduction

associated with an intervention is of minimal use without a baseline incidence of bad outcomes.

You need to have an absolute risk reduction to calculate number needed to treat. (NNT = 1/ARR)

For undesired effects of treatment, calculate the absolute risk increase (ARI), and the number needed to harm (NNH = 1/ARI)

Back-of-the-envelope CEA: Treatment cost per bad outcome prevented = Treatment Cost x NNT

*With regard to dichotomous outcomes

1. Hip replacement vs. screws for hip fracture 2. Arthroscopy vs. Immobilization for 1st time shoulder dislocation3. ApoA-I Milano vs. Placebo to reduce atheroma volume after

acute coronary syndrome4. ISIS-II5. Intravascular gamma radiation vs. placebo to prevent re-

blockage after cleaning out a blocked coronary artery bypass graft.

6. Nimodopine vs. placebo to reduce cerebral infaction after subarachnoid hemorrhage.

7. Oseltamivir to prevent influenza in household contacts of patients with the flu

8. Letrozole (Femara®) vs. placebo to prevent breast cancer recurrence after 5 years of tamoxifen therapy.

Flu Prophylaxis? Pt is a 6-year-old girl with fever, myalgias, cough

and sore throat X 1 day Should you rx prophylactic Tamiflu® for the pt’s

mother who is pregnant?

What if the 6-year-old doesn’t have the flu?

The relative risk reduction is the same (89%), but the baseline risk for the mother is so low that prophylactic oseltamivir doesn’t do much. (ARR is negligible, NNT is enormous.)

Prophylactic oseltamivir works if the index case has the flu, but you don’t know whether she does. You know that 45% of similar patients have laboratory proven influenza.

Probability of Flu + = 45%

NNT is calculated for patients with a particular condition “D”.

P = probability of that condition “D” in your patients

Your NNT* = NNT / P

Note that NNT* goes up as P = probability of condition “D” goes down.

Patient May Not Have The Condition That You’re Treating

*Assumes that treatment for patients without condition D has no value. (For D- patients, ARR = 0, NNT = ∞)

NNT if index case flu + = 9.Probability of flu = 0.45

NNT* = 9 / 0.45 = 20

Probability of Flu + = 45%

*Assumes that treatment when index case is flu - has no value (ARR = 0, NNT = ∞)

No Adjustment of NNH for Disease Prevalence

Oseltamivir is no less likely to cause nausea in household contacts of Flu- patients than in those of Flu+ patients.

Prevalence of Flu+ = P = 45%ARI for nausea is still: 5.5% - 2.6% = 2.9%But, ARR is now:

0.45(12.6 %-1.4%) = 5%

Cases of nausea caused for each case of flu prevented:2.6%/5% ≈ 1/2

BOTE CEA: Oseltamivir

Index Case Flu +• NNT = 9 (Treat 9 household contacts, prevent 1

flu case)• NNT x Treatment Cost* = 9 x $50 = $450/flu

case prevented

45% Prob Flu+• NNT* = 9/0.45 = 20• NNT* x Treatment Cost = 20 x $50 = $1000/flu

case prevented

•Cost of Tamiflu 75 mg #10 = $92.99 www.drugstore.com 11/4/09

Treatment Threshold Probability from Cost-Effectiveness Threshold

IF you can specify the amount you are willing to spend on prophylactic oseltamivir to prevent one case of the flu, you can calculate the treatment threshold probability Ptt

Treatment Threshold Probability from Threshold Cost Effectiveness Ratio

Assume it is worth $2250 in oseltamivir to prevent one case of the flu. We can calculate the treatment threshold probability PTT:

NNT* x $50 = $2250NNT/ PTT = $2250/$50 = 45

NNT/45 = PTT

9/45= 0.2 = PTT

Review: Testing Thresholds (Chapter 3)

Probability of Disease

Treat none; no test

Treat based on test results

Treat all; no test

No Treat-Test Threshold

Test-Treat Threshold

Treatment Threshold

Calculate Test-Treat Threshold

Rapid Antigen Detection Test Sensitivity = 0.5 Specificity = 1.0 PTT =0.2 Ignore cost of test (Imperfect

but costless)

Cost-Effectiveness Threshold Example

Willing to spend $2250 on prophylactic oseltamivir per case prevented.

Ptt = $50/(11.2%×$2250) = 0.20

Using the Chapter 3 notation:C = $50B = (11.2% × $2250) - $50 = $200C/(C+B) = 50/(200+50) = 0.20

Estimating B (using only treatment costs)

V = Value of preventing ONE case (“worth $2250 in oseltamivir to prevent one flu case”)

C = Cost of treating one D- individual unnecessarily (household contact of a Flu- individual) ($50)

B = Cost of failing to treat one D+ individual (household contact of a Flu+ individual)

B = ARR × V – C

V = $2250 (worth $2250 in oseltamivir to prevent one case of the flu)

C = $50 (cost of txing one pt with oseltamivir)

ARR = 11.2% B = 11.2% x $2250 - $50 = $200

Estimating B (using only treatment costs)

Treatment Threshold Probability from B and C

C = $50B = $200B/C = 1:4 = Threshold OddsB/(B+C) = 1/5 = Threshold Prob =

Assumptions

Only considers drug/treatment costs, not side effects of treatment

Treatment does not reduce outcome risk among D- patients

Can specify the amount willing to spend in treatment costs to prevent one outcome

1 Absolute Risk Reduction, Number Needed to Treat, Back-of-the-Envelope Cost Effectiveness Analysis, Treatment Thresholds Revisited 1 November 2012 Michael.

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