A few studies have evaluated higher doses or the administration or a loading dose, which is routine in adults, in pediatric patients, and reported improved colistin exposure without an increased risk of nephrotoxicity The main questions it aims to answer are What is the optimal dosing strategies of intravenous colistin for the treatment of multidrug-resistant gram-negative bacterial infections in preterm neonates? What is the incidence of AKI? What is the factors increasing AKI incidence? A single center retrospective and comparative study, cohort study compare low dose 5 mg/kg/day versus 7.5mg/kg/day

Retrospective study to measure safety and efficacy of colistin conventional dose versus high dose then prospective design to measure pharmacokinetics parameter of both doses to design optimal dose

Inclusion Criteria:

• Neonates aged between (0-30 days) born before 37 weeks
• Critically ill patients with nosocomial infection with proven culture-resistant gram-negative bacteria.
• Neonates who are indicated for colistin and started colistin therapy for at least 48 hr.

Exclusion Criteria:

• Serum creatinine ≥1.5 baseline before colistin
• Received colistin before NICU stay
• Administration of concurrent nephrotoxic drugs including amphotericin, gentamicin, or amikacin.
• Major congenital anomalies or with previous renal impairment.

Preterm, critically-ill, neonates diagnosed with resistant gram-negative infections