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Abstract
The prevalence rate of neonatal jaundice can reach 80%, of which 5% may develop dangerous hemolytic jaundice. The blood test for obtaining bilirubin and hemoglobin concentration is the gold standard for diagnosing hemolytic jaundice; however, frequently drawing blood from jaundice neonates for the screening purpose is not practical. We have developed a handheld diffuse reflectance spectroscopy system to noninvasively determine the bilirubin and hemoglobin levels in neonates. Our study showed that the correlation coefficients were 0.95 and 0.80 for bilirubin and hemoglobin between the results from the blood tests and our handheld system, respectively. This handheld system could be an effective tool for screening hemolytic jaundice.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Neonatal hyperbilirubinemia (NHB), defined as a total serum bilirubin level > 5 mg/dL [1], is a common condition that frequently occurs in neonates. While physiologic NHB could typically resolve itself without treatment in 2-3 weeks, physiologic NHB also could cause severe complications if active intervention is not performed [2]. Regular monitoring of neonatal bilirubin levels using blood sampling or optical noninvasive method is useful for preventing hyperbilirubinemia [3]. Existing noninvasive transcutaneous bilirubinometers are convenient tools for screening neonatal jaundice. However, limitations of these devices, such as overestimation of bilirubin levels in dark-skinned neonates or underestimation of bilirubin levels when TSB was >10 mg/dL, have been reported and thus blood sampling is still the gold standard method for bilirubin level determination [4–7].
NHB induced by hemolytic disease of newborn has an incidence rate between 0.3 to 5% [8–10]. This type of pathologic NHB results from incompatibilities between maternal and fetal blood types, Rh and ABO for instance, and usually appears within 24 h after birth. Hemolytic jaundice is the most serious cause of hyperbilirubinemia among neonates. It may develop to kernicterus which would damage the nervous system, intelligence, hearing or even death of newborns if not treated in time [11]. Cherepnalkovski et al. found significantly lower mean values of hemoglobin and hematocrit in the group with hemolytic NHB than in those with nonhemolytic NHB [12]. They pointed out that hematological parameters (hemoglobin, erythrocytes, and hematocrit) are simple diagnostic methods that assist in the etiological diagnosis of NHB. Therefore, simultaneous monitoring of bilirubin as well as hematological parameters is essential for a complete, effective NHB management. In clinical practice, blood sampling method is used for determining bilirubin and hematocrit levels of neonates. However, it is impractical to check the bilirubin and hematocrit levels of high-risk infants via blood tests frequently. Thus, developing a reliable non-invasive device for neonatal bilirubin and hemoglobin concentration quantification would be beneficial for physiologic and pathologic NHB monitoring, especially in limited healthcare resource settings.
Diffuse reflectance spectroscopy (DRS) systems have been proven to be capable of recovering the absorption and reduced scattering spectra of skin accurately and efficiently [13], and the recovered absorption spectra can then be fitted to the known absorption spectra of the main chromophores appropriately to determine the tissue chromophore concentrations, such as oxygen, hemoglobin, bilirubin, and melanin [14]. In our previous study, we developed a benchtop Diffuse reflectance spectroscopy (DRS) system to perform reliable neonatal bilirubin concentration determination with a diffusing probe using the modified-two-layered (MTL) photon diffusion model. The correlation coefficient of transcutaneous bilirubin (TcB) values recovered by our benchtop system with the total serum bilirubin (TSB) values was 0.92. Moreover, when the TSB was greater than 19 mg/dL, at which level we found that the commercial transcutaneous bilirubin meter displayed “out of range” reading [15].
In this study, based on the configuration of our benchtop DRS system, we built a compact, handheld DRS system for easier operation in the clinical setting. In addition, since the absorption peaks of bilirubin and hemoglobin are at 455 nm, and 540 nm, 556 nm, 576 nm, respectively [16,17], in this study we employed the measurement wavelength range to 450–600 nm so that both bilirubin and hemoglobin concentrations of neonates can be simultaneously determined. The performance of the handheld DRS system was verified using tissue simulating phantoms and the accuracy of recovered bilirubin and hemoglobin concentrations was evaluated through a clinical study.
2. Materials and methods
2.1 Theoretical models (MTL model)
The MTL model is an extension of the general two-layer diffusion model proposed by Kienle et al. [18]. The geometry of the diffusing probe is illustrated in Fig. 1. The diffusing probe was equipped with a high scattering layer to efficiently diffuse the light source so that we could determine the optical properties of the skin using a photon diffusion model. Moreover, the detector fiber is in contact with the tissue layer so that all detected photons can pass through both the high scattering layer and skin and thus the probe’s sensitivity to the skin optical property variation can be ensured. In a two-layer anisotropic medium system, the diffusion equation can be rewritten as
Fig. 1. Side-view of a diffusing probe placed on tissue. Layer 1 is a manmade slab of known optical properties. The placement of the source fiber and the detector fiber are nonsymmetric, and the detector fiber is in contact with the tissue layer.
The light source ${S_i}$ from the source fiber can be approximated as ${S_1} = \delta ({x,y,z - {z_0}} )$ and ${S_2} = 0,$ where ${z_0} = 1/({{\mu_a} + {\mu_s}^{\prime}} )$. As in the previous subsection, the fluence rate of the diffusion equation system can be solved in the Fourier domain by employing the extrapolated boundary condition that assumes the fluence and flux is continuous at the boundary. In the modified two-layer geometry, the detector was located at the boundary of the first and second layers. The fluence rate at the detector has the following form in the Fourier domain:
Here, $\rho = \sqrt {{x^2} + {y^2}} $ and ${R_{Fres}}(\theta )$ is the Fresnel reflection coefficient for a photon with an incident angle relative to the normal boundary.
The measured skin reflectance spectrum was then fitted to the MTL diffusion model to determine the skin absorption and scattering spectra. The recovered skin absorption spectra could then be fitted linearly with known chromophore absorption spectra having characteristic absorption features in the wavelength range to quantify the skin's composition based on the Beer’s law [14,16,17,19]. In the 450–600 nm wavelength range, we considered hemoglobin, melanin, and bilirubin as the major chromophores of skin [16,17,19], and the Beer’s law can be written as:
Here, C, ε and µa represent the concentration, extinction coefficient and absorption spectrum of chromophores, respectively. Chromophore extinction coefficients or absorption spectrum adopted from the Oregon Medical Laser Center website [16,17,19]. The hemoglobin values (total hemoglobin concentration, CtHb) shown in this study were the sum of CHbO2 and CHb values.
2.2 Handheld DRS system
The configuration of the handheld DRS system is shown in Fig. 2 (a). The setup of the handheld DRS system was miniaturized from the original DRS system designed to acquire diffuse reflectance spectra at multiple source-detector separations for the determination of tissue absorption and reduced scattering coefficients. For example, we employed four LED light sources and a detector configured in the modified-two-layered (MTL) geometry with source-detector separations of 1.44, 1.92, 2.40, 2.88 mm for measuring absorption and reduced scattering coefficients of skin [14,15]. In this study, for the system footprint consideration, the probe design was simplified to have two source-detector separations of 1.44 and 2.40 mm. For the probe configuration of this study, two detector channels and one source channel would be sufficient for collecting two diffuse reflectance spectra for the determination of skin optical properties. However, the study subjects were neonates, and they would move freely. To ensure the signal quality, we added an additional source channel, whose location was symmetric to the other source channel with respect to the detector channels, so that the two diffuse reflectance spectra could be collected at a half spectrometer integration time (reduced from 600 ms to 300 ms). The handheld system consisted of two white light LEDs (MCWHD2, THORLABS, USA) as light sources, two mini-spectrometers (C12880MA, HAMAMATSU, Japan) as the detectors, and a mainboard that integrated component drivers, a microcontroller unit (MCU), an analog-to-digital converter (ADC), and a power supply module into a printed circuit board. The MCU (ATMEGA328P, Atmel, USA) took commands from a laptop computer through the USB port to control the spectrometers’ on/off timing and LED input current. Conversely, the detected spectral data from the two spectrometers were transmitted to a laptop computer through USB connection. The spectral data were further processed to absorption spectra and chromophore concentrations in the laptop computer.
Fig. 2. (a) Schematic of our handheld DRS device configuration. The thicker black lines with or without arrows indicate electrical connections and the color lines represent optical connections via optical fibers. Abbreviations: MCU: Microcontroller Unit; Li-Po: Lithium Polymer; ADC: Analog-to-Digital Converter. (b) The top-view configuration of the probe side. The source-detector fiber separations are both 1.44 and 2.40 mm. The red dashed circles represent the positions of source fibers, and the blue circles represent detector fibers. (c) ∼ (e) The photos of our handheld DRS device, (c) the interior printed circuit board and the source-detector components with aluminum alloy cases; (d) the interior light source-detector holder; (e) the appearance of the device with Spectralon slab.
The top view of the distal probing end of the handheld device is illustrated in Fig. 2 (b). We used 400 µm core plastic fibers to deliver the LED light to the top surface of a Spectralon slab (Labsphere, NH, USA) so that the light could be sufficiently diffused before entering skin. Two detector fibers were employed to collect the skin diffuse reflectance at source-detector separations of 1.44 mm and 2.40 mm.
A photograph of the light source and detector components and the interior printed circuit board design is shown in Fig. 2 (c). The spectral response range of the mini-spectrometer was 340–850 nm with a typical spectral resolution (FWHM) of 12 nm. The LEDs and spectrometers were placed in customized aluminum alloy cases with SMA905 mating sleeves to connect the source and detector fibers. Furthermore, Fig. 2 (d) shows that these components were mounted together as a fixed module inside the device. The dimension of the handheld device was L: 16.8 cm x W: 6.77 cm x D: 9.67 cm. A measurement button can be seen in Fig. 2 (e), that was designed for one-handed operation. A typical measurement would take less than 5 s. The measurement data files were stored in the memory card in the device so that the data could be processed latter to determine the bilirubin and hemoglobin values.
2.3 Clinical study
The clinical study was carried out at the Kaohsiung Veterans General Hospital, Taiwan. The institutional review board approved the study protocol (No. VGSKS18-CT1-22). Written informed consent was obtained from the neonates’ parents before each measurement. Neonates that had undergone blood tests to determine their TSB and/or hematocrit were recruited in this study. The TSB values were determined using a capillary sample total bilirubin meter (APEL Neonates BR-5200P, Japan) and hematocrit (Hct) values were determined using a capillary hematocrit reader. In addition, transcutaneous bilirubin (TcB) levels were determined using a Philips BiliChek (Philips, Netherlands) for reference. All DRS measurements were performed three times at each skin site (forehead and sternum) by a medical laboratory scientist, and the mean of the three measurements was determined. All DRS and transcutaneous bilirubin measurements were completed within 30 min after capillary blood sampling for TSB (with or without Hct, if necessary) measurements.
The TcB contains mainly the contribution of perfused bilirubin in extravascular space and the TSB represents the bilirubin concentration in the intravascular space. Bosschaart et. al. indicated that the contribution of intravascular bilirubin to the measured TcB is less than 1% [20]. Our DRS system measures skin tissue which in general contains blood occupying less than 5% of the probing volume. This fact implies that the raw bilirubin values derived from our DRS are intrinsically not equal to the TSB values, and thus a conversion process is typically needed to map the raw bilirubin values to TSB values. By performing the linear regression between CBilirubin and TSB, we mapped the CBilirubin (µM) to Lbilirubin (mg/dL) using the linear regression formula. Similarly, the CtHb measured by the DRS system is intrinsically not equal to the Hct value obtained by blood sampling. The CtHb (µM) value was converted to the clinically used hemoglobin concentration (g/dL) using a linear regression formula. Here, a linear regression formula was obtained for the DRS-derived CtHb values and Hct/3 values, and the formula was employed to transform CtHb to LtHb. Note that the Hct (%) value is generally considered to be three times the value of hemoglobin (Hb) (g/dl) [21].
3. Results and discussion
3.1 Results of clinical study
Twenty-two full-term neonates were enrolled in this study, and all of them underwent the heel prick blood test to check their total serum bilirubin levels. Only 15 of them also measured their Hct values. The demographic characteristics of all neonates are summarized in Table 1. In this study, all newborns were Asian and the average volume fraction of melanin was 0.76 ± 0.26%. In comparison, our previous research using a similar DRS system indicated that the volume fraction of melanin of Asian adults was in the range of 1.66-1.98% [14]. Each neonate was measured using our handheld DRS system three times and BiliChek at the forehead and sternum, respectively. The results are presented in Fig. 3. As mentioned in section 2.3, we calculated the linear regression formula for the CBilirubin and TSB values, and then use the formula to determine LBilirubin values. The linear regression formula was LBilirubin = 1.98 * CBilirubin – 0.93 for the forehead measurements, and was LBilirubin = 1.95 * CBilirubin – 0.94 for the sternum measurements. CBilirubin is the DRS-derived bilirubin concentration and having a unit of µM and LBilirubin is the serum bilirubin concentration (mg/dL). By performing the linear regression between CBilirubin and TSB, we mapped the CBilirubin (µM) to LBilirubin (mg/dL) using the linear regression formula. The slopes of the two formulas listed above have a unit of mg/µM·dL. The TSB levels of all neonates ranged from 5–12.6 mg/dL, and no neonate received phototherapy treatment before the measurements were performed.
Fig. 3. Total serum bilirubin (TSB), optical bilirubin level (LBilirubin), and transcutaneous bilirubin (TcB) were determined using capillary samples, (a, b) our handheld DRS system (H-DRS), and (c, d) BiliChek at the forehead and sternum in 22 neonates, respectively. r represents the Pearson correlation coefficient with TSB values. CI: Confidence Interval. A p-value greater than 0.05 represents that the fitted model is not significantly different from the model y = constant (no correlation).
Table 1. Demographic characteristics of the study groupsa
The Pearson correlation coefficients (r) between TSB and LBilirubin was 0.864 at forehead, whereas that between TSB and TcB via BiliChek was 0.807 (Figs. 3 (a) and 4 (c)). Moreover, the correlation coefficients at the sternum were 0.953 and 0.916 between TSB and LBilirubin and between TSB and TcB via BiliChek, respectively, in these 22 neonates (Figs. 3 (b) and 4 (d)). In a previous study, we measured 27 neonates using a benchtop DRS system. The correlation coefficients between TSB and bilirubin levels derived from the system were 0.88 and 0.92 at forehead and sternum in all neonates, respectively [15]. All three optical systems, including our DRS systems and Bilichek, showed a relatively higher correlation at sternum measurements than at forehead measurements. Maisels et al. also found a better correlation with TSB when transcutaneous measurements were taken by JM-103 on the sternum (r = 0.95) as compared to measurements on the forehead (r = 0.91) [22]. Possibly as a result of continuous exposure to room light, bilirubin measurements in areas that are not covered by clothes, such as the forehead, may present lower bilirubin values [23]. However, some authors found no differences between measurements taken on the forehead and sternum [24]. In general, all systems showed a good correlation with TSB in all neonates at the two measurement locations.
Figure 4 shows the Bland-Altman plots used to assess the agreement at the sternum between TSB and our handheld DRS system as well as between TSB and the BiliChek. The standard deviations were 0.73 mg/dL and 1.03 mg/dL for our handheld DRS system and the BiliChek, respectively. The mean difference between Lbilirubin and TSB was 0.011 mg/dL at the sternum. Note that the BiliChek measurements at the sternum equally overestimated the bilirubin concentrations (mean difference = 3.5 mg/dL), as observed in previous studies [15]. The limits of agreement (LOA) of the handheld DRS system and BiliChek were -1.42∼1.45 and 1.61∼5.63 mg/dL, respectively. The agreement limits are demonstrated as 95% CI (95% CI = mean ± 1.96 SD).
Fig. 4. Bland-Altman plot and 95% limits of agreement between the conventional blood sampling method and the transcutaneous optical measurement at sternum by (a) the handheld diffuse reflectance spectroscopy (H-DRS) system and (b) the BiliChek.
The linear regression formulas for the CtHb values derived from our handheld DRS system and the estimated total hemoglobin values (Hct/3) were calculated for forehead and sternum measurements. The Hct levels of all neonates ranged from 33–68%; therefore, the estimated hemoglobin concentration values were from 11 to 22.7 g/dL. The linear regression formulas were used to determine LtHb from CtHb. They were LtHb = 1.98 * CtHb – 0.6 for the forehead measurements, and LtHb = 1.72 * CtHb + 2.59 for the sternum measurements. CtHb is the DRS-derived total hemoglobin concentration and has a unit of µM and LtHb is the serum bilirubin concentration (g/dL). The slopes of the two formulas listed above have a unit of g/µM·dL.
The total hemoglobin values derived from the handheld DRS system and the hematocrit values of 15 neonates are shown in Fig. 5. The Pearson correlation coefficients (r) between Hct and LtHb were 0.647 and 0.801 for the forehead and sternum, respectively. The standard deviations were 3.33 g/dL and 2.09 g/dL for measurements at the forehead and the sternum, respectively. It should be noted that we did not conduct cross-validation to verify the generalizability of the linear regression formulas due to the limited number of recruited subjects in this study.
Fig. 5. Hct and optical total hemoglobin level (LtHb) determined using our H-DRS at the (a) forehead and (b) sternum. CI: Confidence Interval. A p-value greater than 0.05 represents that the fitted model is not significantly different from the model y = constant (no correlation).
Compared with the bilirubin measurements, we found that hemoglobin measurements were more influenced by measurement sites. The measurement at the forehead had a weaker correlation than that at the sternum. In addition, the linear regression equation at the forehead had a larger slope, confidence interval and intercept at the sternum than the bilirubin measurement. We suspect that the measured cutaneous hemoglobin concentration is not only dependent on the cutaneous vessel density but also is affected by the probe pressure applied to skin during the measurement. Lim et al. claimed that there is not as much tissue and muscle between the skin and skull on the forehead. When pressure is applied to the forehead, skin tissue above a less massive muscle is pushed directly against the skull, clearing blood immediately [25]. While normal hemoglobin only flows in vessels, bilirubin exists in vessels and accumulates in the skin [26]; therefore, we can press on the baby's forehead or chest gently and watch the yellow tint on the skin without blood. Therefore, we speculate that the optical measurement of bilirubin does not have much impact on pressure compared to the optical measurement of hemoglobin. Thus, compared with bilirubin measurement results, the optically recovered hemoglobin values may depend on measurement position (forehead or sternum) and probe pressure.
In this study, no newborns with hemolytic jaundice were found; therefore, the medical staff did not take a separate test of the hemoglobin value of the newborns, but for the newborns with doubts about jaundice, they would receive heel blood tests for bilirubin and Hct. However, Hct is not always exactly three times the hemoglobin (g/dL) for everyone, especially for newborns with abnormal hematologic conditions [27,28]. This could be another reason why the correlation of the LtHb (r = 0.80) measurement was lower than that of the LBilirubin (r = 0.95) measurement. In the future, we will enroll neonates who need to take hemoglobin tests to investigate the correlation between total hemoglobin concentration and the LtHb derived from our handheld DRS system. While our system is resistant to skin melanin bias and could be used to measure subjects with high pigmentation [19], all the subjects recruited in this study were Asian babies with moderate skin pigmentation, and thus we are unable to investigate the impact of skin melanin on the measurement accuracy of bilirubin and hemoglobin levels. In the future, we will design clinical studies to understand the applicability of our DRS system to different ethnic groups.
4. Conclusion
In our previous study, we developed a benchtop DRS system using a diffusing probe with the MTL model, aiming to provide reliable neonatal bilirubin concentration determination. In this study, we developed a handheld DRS system by miniaturizing the benchtop DRS system and investigated the performance of the device in clinical bilirubin and hemoglobin measurements. The preliminary results show that our handheld DRS system had a good correlation (r = 0.953 and 0.801) with the bilirubin and hematocrit values obtained from heel prick tests. The device could help the medical staff notice the occurrence of the dangerous neonatal hemolytic jaundice early and thereby improving the quality of newborn care. Further studies are warranted to determine the performance of handheld DRS system in measuring bilirubin or hemoglobin concentrations of neonates in various clinical settings.
Funding
Ministry of Science and Technology, Taiwan (MOST 109-2811-E-006-503-MY2, MOST-107-2221-E-006-148, MOST-108-2221-E-006-207-MY3).
Acknowledgments
We would like to show our gratitude to Dr. Jenn-Tzong Chang, Dr Yen-Ting Lin, and all nurses in Sick Baby Room in Kaohsiung Veterans General Hospital for assisting the research.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
References
1. M. L. Porter and M. B. L. Dennis, “Hyperbilirubinemia in the term newborn,” Am. Fam. Physician 65, 599–606 (2002).
2. V. K. Bhutani, A. Zipursky, H. Blencowe, R. Khanna, M. Sgro, F. Ebbesen, J. Bell, R. Mori, T. M. Slusher, and N. Fahmy, “Neonatal hyperbilirubinemia and Rhesus disease of the newborn: incidence and impairment estimates for 2010 at regional and global levels,” Pediatr. Res. 74(S1), 86–100 (2013). [CrossRef]
3. Ş. Ercan and G. Özgün, “The accuracy of transcutaneous bilirubinometer measurements to identify the hyperbilirubinemia in outpatient newborn population,” Clin. Biochem. 55, 69–74 (2018). [CrossRef]
4. B. O. Olusanya, D. O. Imosemi, and A. A. Emokpae, “Differences between transcutaneous and serum bilirubin measurements in black African neonates,” Pediatrics 138(3), e20160907 (2016). [CrossRef]
5. S. Samiee-Zafarghandy, J. Feberova, K. Williams, A. Yasseen, S. Perkins, and B. Lemyre, “Influence of skin colour on diagnostic accuracy of the jaundice meter JM 103 in newborns,” Arch. Dis. Child.: Fetal Neonatal Ed. 99(6), F480–F484 (2014). [CrossRef]
6. V. Bhutani, R. Vilms, and L. Hamerman-Johnson, “Universal bilirubin screening for severe neonatal hyperbilirubinemia,” J. Perinatol. 30(S1), S6–S15 (2010). [CrossRef]
7. W. D. Engle, G. L. Jackson, D. Sendelbach, D. Manning, and W. H. Frawley, “Assessment of a transcutaneous device in the evaluation of neonatal hyperbilirubinemia in a primarily Hispanic population,” Pediatrics 110(1), 61–67 (2002). [CrossRef]
8. M. De Haas, F. Thurik, J. Koelewijn, and C. Van der Schoot, “Haemolytic disease of the fetus and newborn,” Vox Sang. 109(2), 99–113 (2015). [CrossRef]
9. D. Patale, T. Lokhande, and R. Chaudhary, “Statistical model for prediction of ABO hemolytic disease of the fetus and newborn in India,” Immunohematology 37(2), 64–68 (2021). [CrossRef]
10. P.-C. Tsao, H.-L. Yeh, Y.-S. Shiau, Y.-C. Chang, S.-H. Chiang, W.-J. Soong, M.-J. Jeng, K.-J. Hsiao, and P.-H. Chiang, “Long-term neurodevelopmental outcomes of significant neonatal jaundice in Taiwan from 2000–2003: A nationwide, population-based cohort study,” Sci. Rep. 10(1), 11374 (2020). [CrossRef]
11. C. C. Wei, C. H. Chang, C. L. Lin, S. N. Chang, T. C. Li, and C. H. Kao, “Neonatal jaundice and increased risk of attention-deficit hyperactivity disorder: a population-based cohort study,” J. Child Psychol. Psychiatry 56(4), 460–467 (2015). [CrossRef]
12. A. Papazovska Cherepnalkovski, V. Krzelj, B. Zafirovska-Ivanovska, T. Gruev, J. Markic, N. Aluloska, N. Zdraveska, and K. Piperkovska, “Evaluation of Neonatal Hemolytic Jaundice: Clinical and Laboratory Parameters,” Open Access Maced. J. Med. Sci. 3(4), 694 (2015). [CrossRef]
13. S.-H. Tseng, C. Hayakawa, B. J. Tromberg, J. Spanier, and A. J. Durkin, “Quantitative spectroscopy of superficial turbid media,” Opt. Lett. 30(23), 3165–3167 (2005). [CrossRef]
14. S.-H. Tseng, P. Bargo, A. Durkin, and N. Kollias, “Chromophore concentrations, absorption and scattering properties of human skin in-vivo,” Opt. Express 17(17), 14599–14617 (2009). [CrossRef]
15. N.-Y. Cheng, Y. L. Lin, M. C. Fang, W. H. Lu, C. C. Yang, and S. H. Tseng, “Noninvasive transcutaneous bilirubin assessment of neonates with hyperbilirubinemia using a photon diffusion theory-based method,” Biomed. Opt. Express 10(6), 2969–2984 (2019). [CrossRef]
16. S. Jacques, “Melanosome absorption coefficient,” (1998), retrieved https://omlc.org/spectra/melanin/mua.html.
17. J. Li, “Bilirubin absorption coefficient,” (1997), retrieved https://omlc.org/spectra/PhotochemCAD/data/119-orig-abs.txt.
18. A. Kienle, M. S. Patterson, N. Dögnitz, R. Bays, G. Wagnières, and H. van Den Bergh, “Noninvasive determination of the optical properties of two-layered turbid media,” Appl. Opt. 37(4), 779–791 (1998). [CrossRef]
19. S. Prahl, “Optical absorption of hemoglobin,” (1999), retrieved http://omlc.ogi.edu/spectra/hemoglobin.
20. N. Bosschaart, J. H. Kok, A. M. Newsum, D. M. Ouweneel, R. Mentink, T. G. van Leeuwen, and M. C. Aalders, “Limitations and opportunities of transcutaneous bilirubin measurements,” Pediatrics 129(4), 689–694 (2012). [CrossRef]
21. B. J. Bain, S. M. Lewis, and I. Bates, Basic haematological techniques,12th ed., Dacie Lewis practical haematology (Elsevier, 2016), Vol. 4, pp. 19–46.
22. M. J. Maisels, E. M. Ostrea Jr, S. Touch, S. E. Clune, E. Cepeda, E. Kring, K. Gracey, C. Jackson, D. Talbot, and R. Huang, “Evaluation of a new transcutaneous bilirubinometer,” Pediatrics 113(6), 1628–1635 (2004). [CrossRef]
23. C. M. D. Conceição, M. F. P. D. S. Dornaus, M. A. Portella, A. D. A. Deutsch, and C. M. Rebello, “Influence of assessment site in measuring transcutaneous bilirubin,” Einstein 12(1), 11–15 (2014). [CrossRef]
24. S. N. El-Beshbishi, K. E. Shattuck, A. A. Mohammad, and J. R. Petersen, “Hyperbilirubinemia and transcutaneous bilirubinometry,” Clin. Chem. 55(7), 1280–1287 (2009). [CrossRef]
25. L. Lim, B. S. Nichols, N. Rajaram, and J. W. Tunnell, “Probe pressure effects on human skin diffuse reflectance and fluorescence spectroscopy measurements,” J. Biomed. Opt. 16(1), 011012 (2011). [CrossRef]
26. N. Purcell and P. J. Beeby, “The influence of skin temperature and skin perfusion on the cephalocaudal progression of jaundice in newborns,” J Paediatr. Child. Health 45(10), 582–586 (2009). [CrossRef]
27. L. Quintó, J. J. Aponte, C. Menéndez, J. Sacarlal, P. Aide, M. Espasa, I. Mandomando, C. Guinovart, E. Macete, and R. Hirt, “Relationship between haemoglobin and haematocrit in the definition of anaemia,” Trop. Med. Int. Health 11(8), 1295–1302 (2006). [CrossRef]
28. I. A. Carneiro, C. J. Drakeley, S. Owusu-Agyei, B. Mmbando, and D. Chandramohan, “Haemoglobin and haematocrit: is the threefold conversion valid for assessing anaemia in malaria-endemic settings?” Malar. J. 6(1), 67 (2007). [CrossRef]
