For an HIV‑infected adult, at which CD4⁺ T‑lymphocyte counts should primary prophylaxis be started for Pneumocystis jirovecii pneumonia, Toxoplasma gondii, and Mycobacterium avium complex, and what are the recommended regimens?

CD4-Based Opportunistic Infection Prophylaxis in HIV-Infected Adults

Start trimethoprim-sulfamethoxazole (TMP-SMX) double-strength once daily when CD4 drops below 200 cells/µL; add azithromycin 1200 mg weekly when CD4 falls below 50 cells/µL; and ensure Toxoplasma-seropositive patients receive adequate coverage when CD4 is below 100 cells/µL. 1

Pneumocystis jirovecii Pneumonia (PCP) Prophylaxis

Initiation Criteria

• Begin primary prophylaxis when CD4 count falls below 200 cells/µL. 2, 1
• Also initiate prophylaxis regardless of CD4 count if the patient has oropharyngeal candidiasis (thrush) or unexplained fever >100°F for ≥2 weeks. 2, 1
• CD4 percentage <14% is an alternative threshold that should trigger prophylaxis even if absolute CD4 count is >200 cells/µL. 2, 3

Preferred Regimen

• TMP-SMX double-strength (800 mg sulfamethoxazole/160 mg trimethoprim) once daily is the first-line regimen. 2, 1
• This single agent simultaneously provides protection against both PCP and toxoplasmosis in seropositive patients, reducing pill burden. 4, 1
• TMP-SMX also offers ancillary protection against bacterial respiratory infections. 1

Alternative Regimens (for TMP-SMX intolerance)

• Dapsone 100 mg orally daily (provides PCP coverage only, no toxoplasmosis protection). 2, 1
• Atovaquone 1500 mg orally daily (PCP coverage only). 2, 1
• Aerosolized pentamidine 300 mg monthly via Respirgard II nebulizer (least preferred; no systemic or toxoplasmosis coverage). 4, 1

Discontinuation Criteria

• Stop PCP prophylaxis only when CD4 count rises above 200 cells/µL and remains elevated for ≥3 consecutive months while on effective antiretroviral therapy (ART) with virologic suppression (HIV RNA <400 copies/mL). 1, 5
• Emerging evidence suggests that in patients with CD4 counts between 101-200 cells/µL who have fully suppressed viral load (<400 copies/mL), the incidence of PCP is extremely low (0.48 cases per 100 person-years), and discontinuation may be safe. 5, 6
• However, do not discontinue prophylaxis if CD4 is ≤100 cells/µL, as data are insufficient to support safety in this range. 5
• Restart prophylaxis immediately if CD4 falls below 200 cells/µL after discontinuation. 1

Toxoplasma gondii Prophylaxis

Initiation Criteria

• Begin prophylaxis when CD4 count falls below 100 cells/µL in patients who are Toxoplasma-IgG seropositive. 2, 1
• If Toxoplasma serology is unknown, obtain IgG testing immediately when CD4 approaches 100 cells/µL. 1
• Seronegative patients should be retested when CD4 remains <100 cells/µL to detect possible seroconversion. 1

Preferred Regimen

• TMP-SMX double-strength once daily alone provides adequate toxoplasmosis prophylaxis; no additional agent is needed. 4, 1
• This is the same regimen used for PCP, offering dual protection with a single medication. 1

Alternative Regimen (for TMP-SMX intolerance)

• Dapsone 50 mg daily PLUS pyrimethamine 50 mg weekly PLUS leucovorin 25 mg weekly provides combined PCP and toxoplasmosis coverage. 2, 1
• Never use aerosolized pentamidine as sole prophylaxis in this CD4 range, as it does not protect against toxoplasmosis. 1

Discontinuation Criteria

• Stop toxoplasmosis prophylaxis when CD4 rises above 200 cells/µL and remains elevated for ≥3 months on effective ART with virologic suppression. 1
• Restart if CD4 falls below 200 cells/µL. 1

Mycobacterium avium Complex (MAC) Prophylaxis

Initiation Criteria

• Begin primary prophylaxis when CD4 count falls below 50 cells/µL. 2, 1
• Rule out disseminated MAC disease clinically before starting prophylaxis. 1

Preferred Regimen

• Azithromycin 1200 mg orally once weekly is the first-line choice. 1
• Azithromycin offers superior adherence due to weekly dosing and has fewer drug interactions with antiretroviral agents compared to clarithromycin. 1
• It also provides additional protection against bacterial respiratory infections. 1

Alternative Regimens

• Clarithromycin 500 mg orally twice daily (equally effective but more drug interactions with protease inhibitors). 1
• Rifabutin 300 mg orally daily (second-line; requires dose adjustments with most antiretroviral agents and mandates exclusion of active tuberculosis before initiation). 1
• Never combine clarithromycin with rifabutin, as this increases adverse effects without improving efficacy. 1

Discontinuation Criteria

• Stop MAC prophylaxis when CD4 rises above 100 cells/µL and remains elevated for ≥3 months on effective ART with virologic suppression. 1
• Some guidelines use a threshold of >50 cells/µL, but >100 cells/µL is more conservative and widely accepted. 1
• Restart if CD4 falls below 50-100 cells/µL. 1

Contemporary Practice Note

• Some experts now forgo routine MAC prophylaxis when ART is started immediately and rapid viral suppression is achieved, though this remains controversial. 1

Critical Drug Interactions and Safety Considerations

• Rifabutin requires dose adjustments when co-administered with protease inhibitors or non-nucleoside reverse transcriptase inhibitors. 1
• Clarithromycin has significant interactions with protease inhibitors; azithromycin is preferred for MAC prophylaxis due to fewer interactions. 1
• TMP-SMX has minimal interactions with modern integrase inhibitor-based ART regimens (bictegravir, dolutegravir). 1
• Screen for G6PD deficiency before using dapsone or primaquine to avoid hemolytic anemia. 4

Common Clinical Pitfalls to Avoid

• Never delay PCP prophylaxis while awaiting CD4 results in patients with oropharyngeal candidiasis or a prior AIDS-defining illness. 1
• Never use aerosolized pentamidine as first-line prophylaxis when TMP-SMX is tolerated; it lacks systemic coverage and does not prevent toxoplasmosis. 1
• Never discontinue any prophylaxis based on a single CD4 measurement; sustained elevation for ≥3 months is mandatory. 1
• Do not ignore CD4 percentage; discordance between absolute CD4 count and CD4% occurs in 13-25% of patients, and providers often fail to prescribe prophylaxis when CD4 count is >200 but CD4% is <14%. 3
• Never combine clarithromycin with rifabutin for MAC prophylaxis. 1
• Exclude active tuberculosis before starting rifabutin to avoid inducing rifampin resistance. 1

Antiretroviral Therapy Integration

• Initiate ART immediately upon HIV diagnosis, even before genotype results are available, using integrase inhibitor-based regimens (bictegravir/tenofovir alafenamide/emtricitabine or dolutegravir + tenofovir + emtricitabine). 1
• For most opportunistic infections, start ART within 2 weeks of diagnosis. 1
• Effective ART with sustained virologic suppression is the cornerstone of preventing opportunistic infections and allows for eventual discontinuation of prophylaxis. 5, 7, 6