Horn et al developed a simple scale for evaluating a possible drug-drug interaction. It is patterned after the Naranjo scale for adverse drug reactions. The authors are from the University of Washington Medical Center in Seattle.
Goal: to assess the probability of a causal relationship between a potential drug interaction and an event
The object drug is the drug affected by interaction with the precipitant drug/
Parameters:
(1) previous credible reports of the interaction in humans
(2) interaction consistent with known interactive properties of precipitant drug
(3) event consistent with known interactive properties of object drug
(4) event consistent with time course of the interaction
(5) stoppage of precipitant drug (dechallange) without change in object drug stops the event
(6) reintroduction of precipitant drug in conjunction with object drug cause recurrence
(7) reasonable alternative cause (nonadherence, clinical condition, etc)
(8) object drug detected in body fluid in concentrations consistent with proposed interaction
(9) objective evidence of effect on object drug other than concentration
(10) change in precipitant drug causes corresponding change in the interaction (greater with increased dose, less with decreased dose; dose dependent with precipitant drug)
|
Parameter |
Finding |
Points |
|
credible reports in humans |
yes |
1 |
|
|
no |
-1 |
|
|
NA |
0 |
|
consistent with interactive properties of precipitant drug |
yes |
1 |
|
|
no |
-1 |
|
|
NA |
0 |
|
consistent with interactive properties of object drug |
yes |
1 |
|
|
no |
-1 |
|
|
NA |
0 |
|
findings consistent with time course of interaction |
yes |
1 |
|
|
no |
-1 |
|
|
NA |
0 |
|
interaction stops if precipitant drug alone stopped |
yes |
1 |
|
|
no |
-2 |
|
|
NA |
0 |
|
interaction recurs of precipitant drug restarted |
yes |
2 |
|
|
no |
-1 |
|
|
NA |
0 |
|
reasonable alternative cause |
yes |
-1 |
|
|
no |
1 |
|
|
NA |
0 |
|
object drug found in concentrations consistent with proposed interaction |
yes |
1 |
|
|
no |
0 |
|
|
NA |
0 |
|
other objective evidence of interaction effect on object drug (other than concentration) |
yes |
1 |
|
|
no |
0 |
|
|
NA |
0 |
|
dose of precipitant drug dependent interaction |
yes |
1 |
|
|
no |
-1 |
|
|
NA |
0 |
total score =
= SUM(points for all 10 parameters)
Interpretation:
• minimum score: -9
• maximum score: 11
• The higher the score the greater the probability of a drug interaction.
|
Total Score |
Probability of Interaction |
|
< 2 |
doubtful |
|
2 to 4 |
possible |
|
5 to 8 |
probable |
|
9 to 11 |
highly probable |
Specialty: Toxicology
