## 1 Introduction
The direct detection format involves the adsorption of an analyte or the presence of an analyte in close proximity to a SERS substrate, which allows the analyte identification or quantification based on its unique SERS spectral fingerprints (intrinsic SERS signal) [54; 55; 56; 57; 58; 59]. On the other hand, the indirect detection format uses a molecular recognition moiety on a SERS substrate, which capture analytes/biomarkers and bring them closer to the SERS substrate. The successful capture of analytes is then confirmed indirectly by monitoring the intense SERS signal of \"reporter molecule\" associated with the SERS substrate (extrinsic SERS signal) [20; 21; 33; 40]. Reporter molecules are usually a dye or a molecule with high Raman scattering cross section that yield an intense SERS signal. Direct detection has the advantage of high specificity and capability to detect analytes without a known specific recognition element, but the spectral interference from the other components in the biological/sample matrix is a common issue. On the other hand, in indirect detection, spectral interference from the sample matrix is minimal as the analytes are isolated using molecular recognition elements, but it has the disadvantage of not being able to acquire the molecular information of the analyte/biomarker itself. Depending on the diagnostic need (binary identification, quantification, or multiplexing), and the sample matrix, the most appropriate SERS detection format could be selected. In general, SERS-based sensing approaches have become more attractive and advantageous over conventional methods such as fluorescence due to ultra-sensitivity, even up to single-molecule detection, rich spectroscopic information provided on the molecular structure of interest that enable direct biomarker detection, and higher multiplexing capability [8; 34; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70]. Among these advantages, the multiplexed SERS detection, in particular, is very attractive in clinical settings as it allows for the detection of a panel of biomarkers ensuring higher confidence in the diagnosis and poses as a better alternative to the commonly used fluorescence-based multiplexed biosensing. This is because, (i) in comparison to fluorophores, Raman tags or Raman-active molecules are highly photostable, hence leading to stable SERS signal over a much longer analysis time. (ii) Raman/SERS peak widths are much narrower than fluorescence peaks, which leads to better spectral resolution, thereby facilitating higher-order multiplexing (in theory, >10) with spectrally unique Raman reporters, and (iii) different SERS tags used in a multiplexed assay can be detected under single-laser excitation source, but for each fluorophore used in a multiplexed biosensing assay, a separate laser excitation source is required. That means the less sophistication in instrumentation and experimental setup needed for SERS multiplexing is much more appealing when developing biosensors for point-of-care (POC) applications. In this mini-review, we highlight the progress in the development in SERS-based diagnostic assays mainly for bacterial infections and mosquito-borne infections. In each section, the current SERS-based assays for common disease biomarkers (nucleic acid, proteins, and metabolites) followed by pre-sample preparation for SERS analysis and diagnostic device designs are discussed. Throughout the review, analytical figure of merits is provided as a guide to the reader to decide the quantitative and qualitative detection capabilities and limitations of each SERS-based diagnostic assays. Challengesand areas of improvements in designing SERS substrates and devices to transform these SERS-based technologies from the lab bench to the field are also briefly discussed. A summary of the highlighted work outlining the disease, biomarker, type of SERS detection (direct/indirect), transducer, and limit of detection (LOD) is also provided for the readers' benefit in the Table 1.