Why Start Acyclovir Before Neonatal HSV Test Results?

It is incredibly scary to watch your newborn receive intravenous (IV) medication in the NICU, especially before doctors even know what is wrong. If your baby was started on acyclovir (an antiviral drug given through a vein), it is important to know this: Starting acyclovir does not mean your baby definitely has herpes simplex virus (HSV).

Doctors often start this medication as a precaution—called empiric therapy—while they run tests and wait for the results ^[1][2].

The Danger of Waiting

Neonatal HSV is a fast-moving infection that can be difficult to recognize early. Many infected babies do not have a fever or the classic skin blisters, and their mothers often have no known history of genital HSV ^[1][3]. Instead, the early signs can be vague, mimicking other common newborn infections or sepsis ^[1].

If treatment is delayed, the virus can spread rapidly through the baby’s bloodstream (disseminated disease) or to the brain (central nervous system [CNS] disease) ^[3][4]. In these severe forms, the disease can worsen quickly, and delays in treatment are associated with worse outcomes ^[3][5]. A UK surveillance study found that mortality in babies who developed disseminated disease was high, and those who survived CNS disease often faced permanent neurological damage, such as cerebral palsy or developmental delays ^[3].

However, when acyclovir is started promptly, the chances of survival and avoiding severe long-term brain injury improve significantly ^[6]. Because early intervention is one of the most important factors doctors can control, standard medical practice is to start acyclovir immediately if there is meaningful clinical concern ^[2].

What Tests Are Done and Why Not Wait?

Testing for HSV involves a PCR test (which looks for the virus’s genetic material). The care team may take samples of the baby’s blood, cerebrospinal fluid (from a spinal tap), or swabs from the surface of the skin, eyes, and mouth ^[7].

While these tests are accurate, they can take anywhere from several hours to a few days to return ^[1]. During this waiting period, if the baby actually has HSV, the virus could multiply rapidly. The medical team protects the baby by starting treatment right away while the evaluation continues.

Weighing the Risks and Benefits

Acyclovir is an effective medication, but like all drugs, it has risks. The most common concern is acute kidney injury, which is a sudden drop in kidney function ^[8][9]. It can also sometimes lower certain white blood cell counts or cause irritation at the IV site ^[10][11].

However, the NICU team closely monitors the baby’s kidney function, hydration, urine output, and blood counts while they are on acyclovir ^[8]. If side effects occur, they are often reversible when recognized early and managed by adjusting fluids or the medication dose.

What Happens Next?

The medical team will review all test results, the baby’s symptoms, and any maternal risk factors.

• If tests are negative and the team is confident there is no infection: The acyclovir may be stopped after a short precautionary course. (Note: A single negative test does not always rule out HSV, so the team will look at the whole picture before stopping).
• If HSV is confirmed or strongly suspected: The baby will need a full course of IV acyclovir, which typically lasts 14 days for skin/eye/mouth infections, and at least 21 days for disseminated or CNS disease ^[9][12].