bias指什么？低估还是高估？

There are many other sources of error with pulse oximetry. Currently available pulse oximeters continue to function even in the presence of arterial hypotension, although there may be a delayed indication of changes in So2, and readings become less accurate below a systolic blood pressure of 80 mmHg.Anaemia tends to exaggerate desaturation readings: at a haemoglobin concentration of 80 g/L normal saturations were correctly recorded, but there was a mean bias of -15% at a true So2 of 53.6%.Patients with dark skin were previously reported to have accurate readings with pulse oximetry, but some bias has been demonstrated at lower So2 values (,80%).5If fingers or toes are used for pulse oximetry, then nail polish should be removed, as different coloured polishes cause variable decreases in So2 values, with red/purple colours having less effect and darker or green/blue colours causing an average of between 1.6% and 5.5% fall in So2 values.5Acrylic nails have also been shown to cause minor inaccuracies in pulse oximeter readings with some, but not all, instruments.5Finally, technical problems with pulse oximetry such as degradation of the light-emitting diode over time and ‘wear and tear’ of cables in the clinical environment can potentially cause inaccurate readings in a large proportion of oximeters

脉搏血氧仪的误差还有许多其他原因。目前可用的脉搏血氧计即使在动脉低血压的情况下仍能继续工作，尽管SO₂变化的指示可能会延迟，并且在收缩压＜80mmHg时读数变得不那么准确⁵⁰。贫血往往夸大了氧饱和度下降的读数：血红蛋白浓度为80g/L时，可记录到正常氧饱和度，但真实SO₂为53.6%时，记录到的氧饱和度平均偏差15%⁵¹。据报道深色皮肤患者的脉搏血氧仪读数准确，但较低SO₂值（＜80%）时会出现一些偏差⁵²。若用手指或脚趾进行脉搏氧饱和度监测，则应去除指甲油，因为不同颜色指甲油会导致SO₂值不同程度下降，红色/紫色影响较小，而深色或绿色/蓝色会导致SO₂值平均下降1.6%～5.5%⁵³。也证明丙烯酸指甲油可导致某些（但非全部）脉搏血氧仪读数的微小误差⁵⁴。最后，脉搏血氧仪的技术问题，如发光二极管随时间老化，以及临床使用中电缆的“磨损”，可能会潜在地导致大量血氧仪读数不准确⁵⁵。

bias是不是就是应该指“exaggerate desaturation readings（夸大了氧饱和度下降？也就是实际上脉搏氧饱和度没下降那么多）”，也就是说真实SO2为53.6%，但测出来的是53.6%-15%=38.6%

但是查原文

A retrospective evaluation of simultaneous tests of oximeters of various manufacturers in volunteer subjects disclosed greater errors at low saturations in subjects with low hemoglobin (Hb) concentrations. Forty-three pulse oximeters of 12 manufacturers studied over a period of 10 months showed that, at a mean arterial oxygen saturation (SaO2) level of 54.5%, as Hb concentration fell, average pulse oximeter (SpO2) bias increased approximately linearly from 0 at Hb greater than 14 g/dl to about -14% at 8 less than Hb less than 9 g/dl. At SaO2 = 53.6%, the mean bias (SaO2--SpO2) of 13 oximeters of 5 manufacturers averaged -15.0% (n = 43) in a subject with Hb = 8 g/dl, but -6.4% (n = 390) in nonanemic subjects. The additional bias in the anemic subject increased with desaturation. It was 0.13% at SaO2 = 98.5% (n = 13), -1.31% at 87.5% (n = 38), -2.71% at 75.1% (n = 38), -5.18% at 61.3% (n = 26), and -9.95% at 53.6% (n = 41); n is the product of the number of oximeters and number of tests in each saturation range. The instruments that showed the greatest errors at low saturations in nonanemic subjects also showed the greatest additional errors associated with anemia (the range between manufacturers of anemic incremental error at about 53% being from -3.2 to -14.5%) and conformed well to the relationship bias (anemic) = 1.35 x bias (normal) -8.18% (r = 0.94; Sy.x = 3.3%). The error due to anemia was zero at 97% SaO2 and became evident when SaO2 fell below 75%.

SaO2--SpO2= -15.0% ，怎么反而是高估了氧饱和度？