Well Guillermo, 

At what anatomical location are you measuring?

What type of activity is being performed (resting, walking, cognitive training, running, etc.)?

Where did you find this formula as an indicator for 02 saturation? Depending on your NIRS device, you may have a parameter which provide the Tissue Oxygen Index (50-70%). Would that be more of a reliable indicator? 

Sorry to ask more questions than give answers. I would just like to have a better idea of what it is you are trying to achieve before giving you some concrete information. 

However to add to your original question, whether or not 02 saturation can be negative or not, I would say that any negative value would be due one of the following factors:

1.) given that NIRS records all  measured values relative to baseline (starting point =0 or 100% baseline) any negative values would reflect that 02 saturation has dropped below the original baseline starting point. 

2.) if you are looking at absolute numbers, i would ask, what is the range (median) of your values recorded? How often do you encounter negative values? Are they outliers in themselves, or do a sequence of negative integers appear, thereby either negating any outlier or miscalculation effect? 

Hi Michael,

Tks for your answer, median values are around 0.75. However, as i pointed out in my question some values are negative and other prety high, such 6 or even 13.. I am using a N-Back paradigm and i am recording brain activity over the whole cortex ( PF-Parietal). Do u think these values are incorrect?

HI Guillermo,

I am interpretating your equation parameters as HBObserved/HBTotal (just intuition, no idea about this topic). In that hypothetical case, it is not possible to have values negatives or higher than 1. The worst case possible is 0 observations in X Total, wich means a index of 0 and the best case is HBO=HBT therefore index=1.

I do need more information to help you through this.

Thx,

Angel 

Hi Angel,

This would be correct if it can be considered that HbO is always smaller that HbT. However, i am not sure that this always the case..is it?

You cannot compute O2 saturation that way. From what I gathered, you are using the modified Lambert-Beer law to compute 'relative' changes in concentrations from the changes in light intensity. As the previous sentence implies, and Michael Nordine already correctly pointed out, These changes are always *relative* with respect to some moment in time. This means, you can add an arbitrary number to it, and still get the a valid 'relative' change (e.g. an increase of 1 a.u. in oxygenated hemoglobin, no matter if your values are 2 and 1, or 102 and 101). Computing the ratio between HbO and HbT (which is HbO+HbR) is dependent on the absolute values (because 1/2 is different than having added 100 to both, i.e. 101/102). This 'arbitrariness' is an inherit characteristic of the modified Lambert-Beer law and thus cannot be circumvented. Hence, there is no way to get the absolute oxygen saturation for these measurements (assuming this is what you did). For example, you can also get different HbX values if you pump up your laser power, resulting in more light reaching the receiver. In absolute terms, however, there should have been no change.

In order to measure absolute oxygenation, you need a different technique. We use the tissue saturation index (TSI), which for example our PortaLite devices can measure, but also our versatile, multichannel Oxymon devices are able to do. See e.g.

http://www.artinis.com/portalite/

http://www.artinis.com/oxymon/

Feel free to contact me/us for more information.

Btw, I added the 'arbitrary value' example to answer your last question: No, HbO is not always smaller than HbT, because you can also subtract an arbitrary large offset from HbR, making the 'relative' change in HbR still valid, but leading to HbT being smaller than HbO. Most devices and/or toolboxes actually do baseline with respect to the first recorded sample.

I thank you for this clear explanation Jörn.