Can I start tenofovir now while awaiting HIV test results in a hepatitis B surface antigen‑positive patient who is about to begin disease‑modifying therapy for myelodysplastic syndrome?

Can You Start Tenofovir While Awaiting HIV Results in an HBsAg-Positive Patient?

Yes, you should start tenofovir (or entecavir) immediately in this HBsAg-positive patient about to begin disease-modifying therapy for myelodysplastic syndrome, even while awaiting HIV test results. The critical imperative is preventing HBV reactivation, which carries a 10-50% risk without prophylaxis and is associated with higher mortality in hematologic patients undergoing chemotherapy 1.

Rationale for Immediate Initiation

All HBsAg-positive hematologic patients, regardless of HBV DNA levels, should receive anti-HBV drugs before or concurrent with starting chemotherapy 1. The guidelines are unequivocal:

• Antiviral prophylaxis should be initiated at least 1 week before or in concomitance with starting chemotherapy 1
• The use of third-generation antiviral drugs (entecavir or tenofovir) is recommended in HBsAg-positive hematologic patients regardless of HBV DNA levels (strong recommendation, moderate quality of evidence) 1
• Without antiviral prophylaxis, the incidence of HBV reactivation in HBsAg-positive hematologic patients undergoing chemotherapy is 10% to 50% and is associated with higher mortality rates 1

Why Tenofovir is Safe to Start Before HIV Results

Tenofovir has activity against both HIV and HBV, but starting it alone while awaiting HIV results does not compromise future HIV care in this specific clinical context:

• The immediate risk of HBV reactivation with chemotherapy far outweighs theoretical concerns about HIV resistance 1
• If HIV testing returns positive, you can immediately add appropriate antiretroviral agents to create a fully suppressive HIV regimen 1
• Tenofovir plus emtricitabine (or lamivudine) is the recommended backbone for HIV-HBV coinfection anyway 1, 2

The FDA label for tenofovir explicitly states: "HIV antibody testing should be offered to all HBV-infected patients before initiating therapy with tenofovir disoproxil fumarate. Tenofovir disoproxil fumarate should only be used as part of an appropriate antiretroviral combination regimen in HIV-infected patients" 2. However, this guidance assumes elective treatment timing—your patient requires urgent prophylaxis before chemotherapy, making this a medical emergency that supersedes the usual sequencing.

Preferred Regimen

Start tenofovir disoproxil fumarate 300 mg once daily immediately 1, 3, 2:

• Tenofovir and entecavir are preferred to lamivudine due to high genetic barriers to resistance (entecavir: strong recommendation, high quality of evidence; tenofovir: strong recommendation, moderate quality of evidence) 1
• Lamivudine is hampered by YMDD mutations, with HBV reactivation (breakthrough) rates of 20-30% in HBsAg-positive lymphoma patients 1
• Both tenofovir and entecavir achieve virological remission (undetectable HBV DNA) in >90% of treatment-adherent patients 3

Critical Action Steps

Immediate actions while awaiting HIV results:

1. Start tenofovir 300 mg daily now (or entecavir 0.5 mg daily as alternative) 1, 3
2. Send HIV antibody/antigen combination test if not already done 1, 2
3. Check baseline HBV DNA, ALT, creatinine clearance, and serum phosphorus 3, 2
4. Counsel patient that if HIV-positive, additional antiretroviral agents will be added immediately 1

If HIV test returns positive:

• Immediately add emtricitabine 200 mg daily plus a third antiretroviral agent (e.g., dolutegravir, bictegravir, or efavirenz) to create a fully suppressive HIV regimen 1
• The combination of tenofovir plus emtricitabine (Truvada) is recommended along with other classes of antiretroviral agents for HIV-HBV coinfection 1
• Do not use tenofovir as monotherapy for HIV—this would select for HIV resistance 1, 2

If HIV test returns negative:

• Continue tenofovir monotherapy throughout chemotherapy and for at least 12-24 months after chemotherapy withdrawal 1

Duration of Prophylaxis

Antiviral prophylaxis must be continued for the duration of chemotherapy and administered for at least 12 to 24 months after chemotherapy withdrawal (strong recommendation, moderate quality of evidence) 1. Subsequent monitoring for delayed HBV reactivation after cessation of antiviral prophylaxis is essential 1.

Monitoring Requirements

During prophylaxis, monitor:

• HBV DNA and ALT every 3 months until undetectable, then every 6 months 3, 4
• Renal function (creatinine clearance) and serum phosphorus at baseline and periodically, especially if risk factors for renal dysfunction exist 3, 2
• In patients with chronic HBV infection receiving antiviral prophylaxis, monitoring for reactivation should be performed before the end of prophylaxis, periodically thereafter, and most likely also during the prophylaxis period 1

Critical Pitfalls to Avoid

Do not delay chemotherapy to "wait for HIV results"—start tenofovir now and adjust the regimen when HIV status is known 1, 4. Do not use lamivudine in this setting due to high resistance rates (20-30% breakthrough in hematologic patients) 1. Do not discontinue antiviral prophylaxis prematurely after chemotherapy ends—continue for at least 12 months post-therapy, as reactivation risk persists 1, 4.