# A Systematic Mapping Study on Research in Anemia Assessment with Non-Invasive Devices
## 4 Results
### Mapping  
This section examines multiple dimensions to identify patterns, trends, or correlations in the research literature. In particular, four charts express interesting information: device type over years (Figure 12); region of interest over years (Figure 13); region of interest vs. device type (Figure 14); device type vs. solution  
The trend charts in Figures 12 and 13 do not include the year 2020 because data from this year, only refers to the months from January to April, so including it in a trend chart would distort the trends making them steeply go down.  
Figure 12 clearly shows how the interest of the scientific community has changed over time, in particular, while co-oximeter research papers decrease, camera-based and smartphone-based research papers increase, it may be associated with the increasing availability and low prices of this kind of devices, thus the increasing interest in the conjunctiva (Figure 13) that is suitable for simple and fast hemoglobin evaluation through more common devices such as cameras and smartphones that also show an increase in popularity. The finger as a region of interest shows a decreasing trend even if  
Figure 14: Region of interest (ROI) vs. device type.  
Figure 13: Region of Interest over Years.  
Figure 15: Device Type vs. Solution status.  
numbers are still high, correlated with the decrease in research interest, and the still massive use of co-oximeters that rely on finger clamps as a source of inputs. An increment is also revealed in other ROIs that are relatively new such as sclera, retina, hand palm, tongue and lip, due to their novelty the number of paper referring to them is yet small, but their popularity is increasing due to the discovery of new non-invasive techniques and devices that allow self-diagnosis.  
The chart in Figure 14 shows that proposed solutions that analyze the conjunctiva are all based on photos of it; most of them use smartphone cameras to take the picture, though only a few go further by embedding the measurement in an app addressed to self-care.  
Fingertips are certainly the most used ROI for non-invasive and mini-invasive measurements, which are mostly made with co-oximeters and similar proposed devices; some studies acquire data from this ROI using cameras (3) or smartphones (4). Papers using sclera, retina, palm, tongue, and nail are all using cameras. This data merged with the trend of these ROIs shown in Figure 13, marks the increasing importance that image analysis techniques are gaining in non-invasive diagnosis. This approach lets researchers easily experiment with new ROIs without requiring specialized hardware, and aims to build a new generation of wide-spread, inexpensive tools.  
The following chart (Figure 15) underlines that no camera and smartphone solutions are on the market.  
Co-oximeters are the only kind of non-invasive device already on the market, although a lot of papers propose improvements to this technology: 13 papers published in 2019 have been identified as setting a proposal to improve measurements by the use of optical sensors. The main trends to enhance co-oximetry are the use of artificial intelligence approaches [40; 41; 42; 43; 44; 45; 46], find the optimum wavelengths [40; 41; 42; 43; 47; 48; 49; 50; 51; 52], noise reduction [45; 46], and minimize illumination influence [45].