The Brief Case: Confirmed Positive HIV-1 Serologic Screening but Undetectable RNA Virus Load in a Pregnant Woman

DISCUSSION

HIV screening is routine and broadly indicated. The U.S. Preventive Services Task Force recommends screening for individuals between 15 and 65 years of age, other individuals with risk factors, and all pregnant women, including those who present in labor. In the United States, the prevalence of HIV is low, with an estimated 299.5 per 100,000 individuals living with HIV infection in 2014 (1). As a result, false-positive results are common during initial screening, and the U.S. Centers for Disease Control and Prevention (CDC) recommends a multistep algorithm. Widely used fourth-generation screening tests detect HIV-1 p24 antigen and IgM and IgG antibodies to HIV-1 and HIV-2. Positive HIV screening results are confirmed with an HIV-1/HIV-2 antibody differentiation assay, and any test discrepancies are resolved through RNA nucleic acid amplification testing. In our case, the positive results were further confirmed by HIV-1 Western blotting showing antibodies to proteins encoded by the three main HIV genes, gag, pol, and env. Particularly surprising, then, was the lack of HIV-1 RNA detectable in the patient's plasma by either RT-PCR or TMA.

Our case raises three main learning points. First, while false-positive results of HIV antibody screening tests of pregnant women are regularly encountered in the practice of laboratory medicine, it is important to remember that a positive screening result may also represent a true HIV infection, even in a patient without apparent risk factors (2). Second, this case was further complicated by the absence of HIV-1 RNA in the patient's plasma, as confirmed by multiple nucleic acid amplification methods, although we cannot formally exclude the possibility that these methods could not detect RNA because of mutations in the targeted primer or probe regions of the patient's HIV-1 strain. Though plasma HIV-1 RNA testing is recommended as the final step in the HIV diagnostic algorithm, this case highlights the utility of whole-blood proviral HIV-1 DNA testing to confirm a diagnosis of HIV infection, particularly in patients with low or undetectable viral RNA levels in their plasma. Finally, the laboratory test results presented here indicate that this patient mounted an immune response that suppressed viral replication, even in the absence of ART.

Positive antibody test, negative RNA test, and positive proviral HIV DNA test results suggest that the patient may be a long-term nonprogressor (LTNP). This term has been used historically to describe a heterogeneous population of patients with light virus loads and CD4 T cell counts in the normal range in the absence of ART. This includes patients with the delta CCR5 mutation, which prevents CCR5-tropic virus from fusing to cells, and patients who develop early neutralizing antibody responses to HIV. The contemporary use of LTNP, however, refers to so-called elite controllers (ECs) whose immune systems suppress viral replication without the need for ART (3).

Homozygosity for the delta CCR5 mutation has been identified in high-risk HIV-exposed individuals who remain seronegative, suggesting that HIV is not able to enter, integrate, or replicate in cells (4). Heterozygosity for delta CCR5 has also been reported to delay the progression to AIDS, but these patients have detectable plasma virus loads (5). Broadly neutralizing antibody responses are associated with lighter virus loads, and neutralizing antibodies may develop early after infection in some individuals, who have been termed “elite neutralizers.” However, the presence of neutralizing antibodies does not correlate well with patients with no detectable virus load (6). Given that our patient has evidence of integrated virus with an undetectable virus load, she most likely is neither a carrier of the delta CCR5 mutation nor an elite neutralizer.

The most plausible explanation is that our patient is an LTNP-EC. Various definitions of LTNP-ECs have been proposed, but in one study, defining this population by using consecutive HIV-RNA measurements of <75 copies/ml with a follow-up of ≥6 months or with 90% of measurements at <400 copies/ml over a ≥10-year follow-up performed best in classifying nonprogression of disease (7). Using this definition, the time to disease progression, as defined by AIDS, death, initiation of ART, or a CD4 T cell count of <350, still varies widely, from approximately 1 to 25 years (7). These patients may have undetectable virus loads, while proviral DNA is usually still detectable, consistent with our patient's findings (8). As we have results from only a single time point with an undetectable virus load and our patient chose to begin ART, definitive declaration that she is an LTNP-EC according to the above definition is not possible. Nevertheless, lack of transmission of HIV to her husband over an approximately 18-month period suggests that her virus load remained at low levels during this period.

Several factors other than HIV infection (e.g., medical comorbidities, pregnancy, infections, medications, and alcohol abuse) affect the CD4 T cell count and explain its significant variability upon laboratory testing. In healthy individuals, the CD4 T cell count falls in the range of 800 to 1,050/μl, corresponding to 28 to 57% of the absolute lymphocyte count. A significant change usually implies a 30% change in the absolute count or a 3-percentage point change in the CD4 percentage. LTNP-ECs often maintain normal CD4 T cell counts despite a lack of ART, with median counts ranging from 725 to 903/μl reported in seven LTNP-EC cohorts (9). Our patient's CD4 T cell count before initiation of therapy was 404/μl, below the intraquartile range reported in these cohorts. However, even after initiation of ART, our patient's CD4 T cell counts ranged from 395 to 750/μl, with a CD4 T cell count of 496/μl 21 months after ART initiation, despite optimal adherence to therapy. Declines in CD4 T cell counts have also been described in LTNP-ECs with virus loads consistently <50 copies/ml and are thought to be due to increased immune activation rather than increased viral replication (10).

A final clue to her infection course may be in her HLA-B*5701-positive status, which was determined prior to initiation of therapy to identify hypersensitivity to abacavir. Among LTNP-ECs, the HLA-B*5701 allele is overrepresented, with a frequency of 65% reported in the largest LTNP-EC cohort (9). The reported frequency of B*5701 in the South Asian population is approximately 8%, and the frequency of LTNP-EC in the population of HIV-infected individuals is estimated to be 0.3% (11, 12). According to the Bayes rule, one would expect that of persons who are HLA-B*5701 allele carriers, 2 to 3% would be ECs.

In conclusion, we present a case of a likely LTNP-EC in pregnancy diagnosed with HIV-1 through routine serologic prenatal screening. Undetectable HIV-1 RNA in the patient's plasma confounded the diagnosis and raised concerns about false-positive results in antibody testing. However, whole-blood proviral DNA testing confirmed the diagnosis of HIV-1, demonstrating the utility of this test in helping to resolve difficult cases in which the standard HIV diagnostic algorithm is indeterminate.