Mo. Rev. Stat. §376.383(6)(1)

(6)(1) If the health carrier has not paid the claimant on or before the forty-fifth processing day from the date of receipt of the claim, the health carrier shall pay the claimant one percent interest per month and a penalty in an amount equal to one percent of the claim per day. On claims where the amount owed by a health carrier exceeds thirty-five thousand dollars on the unpaid balance of a claim, the health carrier shall pay the claimant one percent interest per month and a penalty in an amount equal to one percent of the claim per day for a maximum of one hundred days, and thereafter shall pay the claimant two percent interest per month. The interest and penalty shall be calculated based upon the unpaid balance of the claim as of the forty-fifth processing day. The interest and penalty paid pursuant to this subsection shall be included in any late reimbursement without the necessity for the person that filed the original claim to make an additional claim for that interest and penalty. A health carrier may combine interest payments and make payment once the aggregate amount reaches one hundred dollars.