PCR is a highly sensitive method for detecting VZV [17,18,19]. PCR is useful to detect VZV DNA in the blood [including whole blood, serum, plasma, peripheral blood mononuclear cells (PBMC)], saliva, and cerebrospinal fluid (CSF) of patients for the early diagnosis of atypical HZ in patients without a rash [20,21,22,23]. VZV is detectable in the blood from 5 days before eruption to 4 days after eruption, while VZV in T lymphocytes is detectable as early as 8–10 days before eruption and persists beyond six months [24]. In whole blood, approximately 60% of VZV is present in PBMC and 40% in serum or plasma, and the DNA positivity rate of VZV in PBMC samples detected using PCR is higher than that in plasma and serum samples in patients with HZ. Whole blood is more sensitive for the detection of VZV DNA than PBMC, possibly because it contains VZV DNA from lysed cells [25, 26]. The viral load gradually decreases with treatment of the disease, and real-time PCR can help monitor the effectiveness of treatment in patients with HZ.

Collecting saliva from patients and detecting VZA DNA is more convenient and readily available than collecting plasma. Mehta et al. detected VZV DNA in the saliva of 54 shingles patients (100%) on the day of rash onset [27]. In a study including 70 patients with first-time HZ detected using PCR testing, saliva VZV was detected in 85.7% of patients, and the positive rates on days 1, 8, 15, and 29 were 85.7%, 47.6%, 19.2%, and 23.1%, respectively [28]. Another study reported that in patients with suspected shingles, the sensitivity of salivary DNA PCR for VZV detection (88%) was significantly higher than that of plasma DNA PCR (28%), with no difference in specificity [29]. Nagel et al. detected VZV DNA using PCR in the saliva of a patient with a 12-year history of HZ, indicating that VZV DNA can persist in saliva samples for a long time [30].

CSF samples from patients infected with VZV in the central nervous system were detected using PCR, and the DNA positivity rate of VZV was higher than that of plasma samples and serological detection methods. Grahn et al. retrospectively analyzed 72 patients diagnosed with central nervous system (CNS) VZV infection and neurological symptoms using cerebrospinal fluid VZV DNA testing. Real-time PCR has been used to detect higher levels of VZV DNA in CSF samples than in serum samples [31]. In a study including 34 patients with VZV-infected central nervous system, VZV DNA (67.6%) and VZV-IgG antibodies (32.4%) in the CSF were detected using PCR and enzyme-linked immunosorbent assay (ELISA), respectively [32].

It is meaningful for PCR detection to collect blood and saliva from patients with suspected herpes zoster without eruption. These studies show that VZV DNA is more detectable in saliva than in blood. CSF testing is an invasive procedure that is not suitable for ordinary patients with shingles.

When VZV is reactivated, serum VZV antibody titers remain high and significantly different from those of healthy controls. Serum antibody levels are strongly associated with shingles development, and measurement of serum VZV antibody titers is helpful in the diagnosis of shingles [33, 34]. Kangr et al. used IgM antibody-capture radioimmunoassay (MACRIA) to detect IgM antibodies in the serum of 220 patients, of whom 216 (98%) were diagnosed with shingles, of which 94.4% of serum samples were acquired 2–6 weeks after the rash onset [35]. Serum samples of typical patients with HZ were collected, and 37% of patients with HZ were identified as positive for VZV-IgM antibody with VZV-IgM titer using ELISA. Subsequently, VZV-IgM titers of positive patients were analyzed, and the results revealed that VZV-IgM titers started increasing after the appearance of skin lesions, reached the highest level 6–10 days after the rash onset, and was negative in all patients after 10 weeks. Therefore, VZV-IgM titers are meaningful to detect within 3.5 weeks after the onset of symptoms [36].

In 141 patients with HZ, the positive rates of VZV-IgG antibody, VZV-IgM antibody, and complement fixation (CF) test were 93.9%, 12.0%, and 64.2%, respectively, among which VZV-IgG antibody had a strong correlation with CF titers, and the CF titer largely represented the IgG titer. CF titers tend to increase slowly over time and are weakly correlated with time of onset [37]. In a study including 865 shingles patients who presented with the incidence of VZV-specific CF at their initial presentation to a dermatological clinic, 66% of patients with HZ showed negative CF. Subsequently, paired complement binding tests performed over a short period of time showed a significantly elevated titer, peaking approximately 2 weeks after the onset, and gradually decreasing after 1 year [38].

IgM appears first but does not last long and is a marker of recent infection; therefore, VZV⁃IgM antibodies testing is only beneficial in confirming the diagnosis of acute-phase shingles. The relationship between complement titer and time of VZV onset remains controversial and requires further elucidation. IgG appears and persists at the initial infection with the VZV virus and is of little significance in the early diagnosis of shingles.

VZV is caused by the dorsal root ganglia, which causes nerve root inflammation and changes in cytokine levels in the body. Galectin-3 is a member of the β-galactose-bound lectin family, secreted by monocytes, phagocytes, and epithelial cells, and is involved in biological processes, such as cell interactions, cell cycle, regulation of cell growth, splicing of mRNA precursors, and angiogenesis. After VZV infection, the mRNA and protein expression of galectin-3 in the dorsal horn of the spinal cord of mice was significantly increased. Galectin-3 gene deletion in mice, or intrathecal injection of galectin-3 antibody, significantly reduced tactile pain, suggesting that galectin-3 is involved in PHN production [39]. Wang et al. reported that plasma galectin-3 and IL-6 levels in patients with HZ neuralgia were significantly higher than those in healthy physical examiners, and galectin-3 levels could be used as a new biochemical marker for patients with pre-HZ and PHN [40, 41].

Reduced immunity is a predisposing factor for shingle development. The immune response to shingles is thought to be dysfunctional and mainly manifested by specific cellular immunosuppression [42, 43]. In a case analysis including 83 patients with multiple myeloma, patients with multiple myeloma infected with HZ had elevated serum CD3+ and CD4+ levels and significantly lower CD8+ levels after treatment [44]. Jung et al. found that VZV induces a broad immune response by affecting the release of immunoactive substances such as cytokines, neurotrophic factors, and chemokines in vivo, leading to T-lymphocyte immunosuppression [45]. Zhu et al. reported that shingles patients had elevated levels of IL-6 in the acute phase; CD3+ , CD4+, and CD8+ levels were lower than normal and were closely associated with the occurrence of PHN [46]. Many markers of inflammation are used clinically to detect herpetic neuralgia but is rarely diagnosed before eruption. Gal-3, IL-6, and T lymphocytes detected in the above study were all conducted in patients with post-diagnosis herpes zoster. Studies on differences in blood levels of inflammatory factors between pre-eruption and healthy patients are lacking. The specificity of these inflammatory factors for the diagnosis of shingles also needs further study.

Wang et al. reported 44 differentially expressed proteins found in the plasma of patients with HZ, and the main pathways in which these molecules were involved included the MAPK signaling pathway, neuroactive ligand-receptor interactions, acute myeloid leukemia, and transcriptional regulation disorders in tumors. Six key molecules were selected as candidate molecules for further study, and plasma from 40 patients with HZ and 40 healthy participants was validated using ELISA, immunoblot assay, and receiver operating characteristic curve analysis. Finally, three proteins, PLG, F2, and VTN, were found and can be used as biomarkers for the detection of early patients with HZ [47].

Non-coding RNA refers to RNA that does not code for proteins, including rRNA, tRNA, snRNA, snoRNA, and microRNA with a variety of known functions, as well as RNA with unknown functions. MicroRNAs are a class of endogenous non-coding RNAs found in eukaryotes with regulatory functions; they are involved in maintaining normal cell function, host-virus interactions, and restricting the replication of certain virus types. In a study including 41 patients with HZ for the detection of microRNAs, serum miRNA levels were analyzed using TaqMan low-density array and confirmed using quantitative reverse transcription PCR (RT-qPCR) analysis. The expression levels of six miRNAs, including miR-190b, miR-571, miR-1276, miR-1303, miR-943, and miR-661, were significantly increased in patients with HZ; therefore, it may be used as a biomarker to detect potential HZ infection [48]. Markus et al. reported VZV-encoded small non-coding RNAs (sncRNAs), with at least one VZV sncRNA expressed in productive infections in neurons and fibroblasts, which may reduce viral replication. Since sncRNAs are considered potential targets for antiviral therapy, identifying these molecules in VZV may provide a new direction for the development of HZ pain treatments [49].

Whether it is proteomic analysis or non-coding RNA research, large samples, multicenter research, and repeated validation are required. In addition, the acquisition of blood samples from patients before eruption is the focus and difficulty of this study. Based on previous studies, our research team will continue to investigate changes in blood protein expression in patients with preeruption.