Clinical case

A man in his 50s with prior orthotopic heart transplantation, chronic kidney disease, and type 2 diabetes presented with a progressive rash. Immunosuppression comprised tacrolimus, mycophenolate mofetil, and prednisone. Admission is designated day 0. A wart-like right-groin lesion had appeared approximately six weeks earlier. Scrotal/perineal lesions developed approximately one month before admission, with crateriform crusting, malodorous drainage, and subsequent spread to the thighs, arms, back, face, and tongue (Figure 1). He denied fever, chills, animal exposure, or new sexual exposures and reported one female partner. He described prior oral HSV and shingles vaccination. Penicillin for reportedly weakly positive outpatient syphilis serology had not improved the rash.

Figure 1.

Cutaneous lesion on the shoulder. The clinical photograph was cropped to exclude potentially identifying features, including a nearby tattoo.

Disseminated VZV/HSV was initially favored. Airborne/contact precautions and intravenous acyclovir preceded the mcfDNA result. Doxycycline/cefepime, followed by amoxicillin-clavulanate/doxycycline, addressed possible bacterial superinfection. Day-0 skin biopsy showed herpesvirus-like dermatitis, negative VZV staining, and nondiagnostic focal HSV-1/2 staining. Negative or unrevealing studies included HIV antigen/antibody, repeat syphilis serology, urine Chlamydia trachomatis/Neisseria gonorrhoeae RNA, plasma HSV-1/2 PCR, scrotal HSV/VZV testing, whole-blood VZV PCR, Coccidioides/Blastomyces serologies, right- and left-arm blood cultures, and skin-biopsy bacterial culture/Gram stain, anaerobic culture, fungal culture, and acid-fast bacillus smear/culture. These studies were obtained during hospitalization; not all final results preceded mcfDNA reporting.

Plasma collected on day 1 yielded a Karius Spectrum report on day 3 identifying “Monkeypox virus” at 49,730 molecules/100 nL and “Human adenovirus D” at 1,402 molecules/100 nL. Monkeypox virus (MPXV) appeared under “Obligate Pathogens,” flagged as an alert. The report defines concentration as unique microbial DNA fragments per 100 nL of plasma, with an assay analytical range of 50–210,000 molecules/100 nL. This quantitative DNA-fragment estimate is not interchangeable with viral genome copies or a validated disease-severity measure. The report provided no MPXV-specific clinical cutoff; its obligate-pathogen category is interpretive, not a zero detection threshold. Adenovirus was not considered explanatory of the rash; this interpretation was clinical, not based on comparison of the two concentrations.⁷

The compatible syndrome and mcfDNA detection supported a clinical diagnosis of mpox. No lesion MPXV PCR or other orthogonal confirmation was performed at any laboratory. Following joint consultation among the treating team, city/state health departments, and the Centers for Disease Control and Prevention (CDC), additional testing was judged unlikely to alter management. The team also wished to avoid another biopsy. This was an individualized decision, not a recommendation against lesion swabbing or evidence that swabbing requires biopsy. Acyclovir was discontinued and public health authorities were notified; isolation was already in place.

On CDC recommendation, the initial plan documented on day 6 addressed progressive, extensive mucocutaneous disease during transplant immunosuppression with combination therapy: oral tecovirimat 600 mg twice daily with food for 14 days and brincidofovir 200 mg weekly for two doses. Mycophenolate was held. The day-4 evening tacrolimus dose was withheld, followed by reduction from 1 mg twice daily to 0.5 mg twice daily on day 5; prednisone 5 mg daily continued. Daily inpatient tacrolimus concentrations ranged from 8.7 to 14.4 ng/mL; the transplant target was 7–9 ng/mL. Acute kidney injury developed during empiric therapy; creatinine peaked at 3.69 mg/dL on day 4 and decreased to 1.79 by discharge on day 7 following intravenous fluids and medication adjustments. Inpatient aminotransferases remained within reference limits; bilirubin briefly reached 1.3 mg/dL. Home-isolation and partner-notification counseling accompanied discharge.

At day-28 follow-up, the initial antiviral course and 10 days of amoxicillin-clavulanate/doxycycline were completed. Facial/nasal lesions had healed, but genital/perineal lesions remained incompletely healed. On day 34, CDC/Food and Drug Administration (FDA) consultation supported approval of another 14 days of tecovirimat and two additional weekly brincidofovir doses. Repeat complete blood count and comprehensive metabolic panel were ordered, and pharmacy coordinated access. The planned cumulative exposure was 28 tecovirimat treatment days and four brincidofovir doses, not a documented uninterrupted course. Completion of this additional course and subsequent laboratory results were not documented in the available follow-up.

Diagnostic stewardship implications

The epidemiologic history and herpesvirus-like histology contributed to anchoring on familiar diagnoses. Nevertheless, compatible morphology and genital/perineal involvement should prompt lesion MPXV PCR regardless of reported partner history or absence of a recognized exposure.^{Reference Khanna, Kost and Wu3,4} Sequencing redirected treatment in this patient; it should not routinely compensate for omitting MPXV from the differential. The actionable changes were antiviral adjustment and public health notification, not initiation of isolation. Improvement during combination treatment does not establish either diagnostic certainty or the incremental benefit of adding brincidofovir. Such treatment requires individualized assessment in immunocompromised patients.⁸

Conventional mpox PCR testing

Lesion-surface swabbing with appropriate personal protective equipment remains recommended; biopsy, unroofing, and sharps are unnecessary.⁴ Avoiding another biopsy was not a contraindication to recommended swab-based PCR. The absence of orthogonal confirmation limits diagnostic certainty despite clinical compatibility and mcfDNA detection. Our case demonstrates a clinically supported diagnosis, not lesion-PCR confirmation, comparative diagnostic accuracy, or superiority of sequencing. Early syndrome recognition, appropriate specimen selection, and laboratory/public-health communication remain the central stewardship interventions.

Plasma mcfDNA sequencing

Karius Spectrum is a laboratory-developed test performed in a laboratory certified under the Clinical Laboratory Improvement Amendments and accredited by the College of American Pathologists; manufacturer materials state that it has not been reviewed or cleared by FDA.^6,7 MPXV detection occurs within its broader pathogen-agnostic sequencing framework. Published MPXV analytical characterization and a 12-patient case series support feasibility but do not establish prospective clinical sensitivity or negative predictive value.^{Reference Park, Lindner and Brick5} Consequently, a negative plasma result cannot exclude mpox.

Reporting depends on assay statistical thresholds rather than a universal clinical concentration cutoff.⁷ The analytical range is not an MPXV reference interval or a validated clinical decision threshold. Concentration must be interpreted alongside the syndrome and cannot establish viability, infectiousness, or treatment response. No serial mcfDNA results were available for this report. Cost, access, and incidental detections further limit routine use. Selected diagnostically unresolved cases may benefit, but sequencing does not replace lesion PCR when mpox is clinically suspected.