The purpose of this study is to measure concentrations of Raltegravir in cerebrospinal fluid.

The hypotheses are:

• Raltegravir concentrations in CSF will be measurable
• Raltegravir concentrations in CSF will remain constant, while plasma concentrations vary widely leading to highly variable CSF-to-plasma ratios.
• Following at least 4 weeks of Raltegravir-containing combination therapy, HIV RNA levels in CSF will be below 50 copies/mL in all subjects.

Inclusion Criteria:

• Enrollment in a raltegravir parent protocol at UCSD.
• Willing to undergo lumbar puncture.
• Able to give informed consent. If cognitively impaired, a test will be administered to ensure adequate comprehension of the consent document and procedure.

Exclusion Criteria:

• Relative or absolute contraindication to lumbar puncture, such as current coagulopathy, thrombocytopenia (platelets<50,000/µL), hemophilia, or use of anticoagulant medication.
• Psychiatric disease that would interfere with study participation, in the opinion of the investigator.
• No major opportunistic infections within 30 days.
• Moderate or severe cognitive impairment. The proposed study does not have a neuropsychological objective.

Participants will be selected from participants already enrolled in Merck-sponsored Raltegravir Clinical Trials.