Table 6. Proposed algorithms in response to measured drug levels in the setting of symptoms of active disease.

6TGN concentration	6MMP concentration	Interpretation	Strategy
In therapeutic window*	Normal or high^	Refractory to thiopurines	Change therapy – can discontinue thiopurine or continue at same dose in conjunction with the new therapy
Low	Low or normal	Too low of dose or noncompliant	Increase dose or educate regarding compliance
High	Normal or high	Refractory to thiopurines	Change therapy and discontinue thiopurine or continue at same dose or lower dose
Low	High	Preferential shunting to 6MMP†	Change therapy or reduce dose and add allopurinol

Anti-Infliximab antibody	Infliximab concentration	Additional testing	Strategy
Positive			Change to another anti-TNF therapy. If persistent disease activity after changing agents, change to agent with different mechanism
	In therapeutic window	Active disease on endoscopy / radiology	Change to agent with different mechanism
	In therapeutic window	Inactive disease on endoscopy / radiology	Investigate for alternative etiology of symptoms
	Sub-therapeutic#		Increase dose or frequency. If persistent disease activity, switch to a different anti-TNF agent.
	Sub-therapeutic#		Alternative strategy - Switch to a different anti-TNF agent. If persistent disease activity, change to agent with different mechanism.

^*Therapeutic window for 6TGN defined as 235–450 pmol/8 _ 108 red blood cells

^{^}Therapeutic level of 6MMP is less than 5700 pmol/8 _ 108 RBC

^†A ratio of 6MMP:6TGN greater than 11

^#Sub-therapeutic infliximab concentration defined as <12 mcg/ml at 4 weeks or undetectable trough level