Causality assessment is the method by which the extent of relationship between a drug and a suspected reaction is established

Currently wide variety of causality assessment scales exist, to attribute clinical events to drugs in individual patients or in case reports, each with their own advantages and limitations. These scales include

Karch & Lasagna scale
Naranjo's scale
WHO probability scale
Spanish quantitative imputation scale
Kramer's scale
Jones scale
European ABO system
Bayesian system.
The Naranjo's scale and the WHO scale of assessment are the most commonly used scales.

NARANJO's ALGORITHM

question Yes No Don't know
Are there previous conclusion reports on this reaction? +1 0 0
Did the adverse event appear after the suspect drug was administered?

+2

 -1 0
Did the AR improve when the drug was discontinued or a specific antagonist was administered? +1 0 0
Did the AR reappear when drug was readministered? +2 -1 0
Are there alternate causes [other than the drug] that could solely have caused the reaction? -1 +2 0
Did the reaction reappear when a placebo was given? -1 +1 0
Was the drug detected in the blood [or other fluids] in a concentration known to be toxic? +1 0 0
Was the reaction more severe when the dose was increased, or less severe when the dose was decreased? +1 0 0
Did the patient have a similar reaction to the same or similar drugs in any previous exposure? +1 0 0
Was the adverse event confirmed by objective evidence? +1 0 0

 

 

 

 

 
Source: A METHOD FOR ESTIMATING THE PROBABILITY OF ADVERSE DRUG REACTIONS - NARANJO et al

SCORING FOR NARANJO's ALGORITHM

• > 9 = definite ADR
• 5-8 = probable ADR
• 1-4 = possible ADR
• 0 = doubtful ADR
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WHO SCALE OF ASSESSMENT

1.CERTAIN A clinical event, including laboratory test abnormality, occurring in a plausible time relationship to drug administration, and which cannot be explained by concurrent disease or other drugs or chemicals. The response to withdrawal of the drug (dechallenge) should be clinically plausible. The event must be definitive pharmacologically or phenomenologically, using a satisfactory rechallenge procedure if necessary.
2.PROBABLE/ LIKELY A clinical event, including laboratory test abnormality, with a reasonable time sequence to administration of the drug, unlikely to be attributed to concurrent disease or other drugs or chemicals, and which follows a clinically reasonable response on withdrawal (dechallenge). Rechallenge information is not required to fulfil this definition.ical event, including laboratory test abnormality, with a reasonable time sequence to administration of the drug, unlikely to be attributed to concurrent disease or other drugs or chemicals, and which follows a clinically reasonable response on withdrawal (dechallenge). Rechallenge information is not required to fulfil this definition.
3.POSSIBLE A clinical event, including laboratory test abnormality, with a reasonable time sequence to administration of the drug, but which could also be explained by concurrent disease or other drugs or chemicals. Information on drug withdrawal may be lacking or unclear.
4.UNLIKELY A clinical event, including laboratory test abnormality, with a temporal relationship to drug administration which makes a causal relationship improbable, and in which other drugs, chemicals or underlying disease provide plausible explanations.
5.CONDITIONAL/ UNCLASSIFIED A clinical event, including laboratory test abnormality, reported as an adverse reaction, about which more data is essential for a proper assessment or the additional data are under examination.
6.UNASSESSIBLE/ UNCLASSIFIABLE A report suggesting an adverse reaction which cannot be judged because information is insufficient or contradictory, and which cannot be supplemented or verified.
Source : WHO