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For investigative purposes, I'm searching for a tool that can be used as a quick visual assessment of the the length of time that blood has been exposed to air after the blood flow has stopped.. Let's say that you walk into a room and see someone unconscious. Where blood appears, it had already stopped any fresh flow & has already dried to a dark brown similar to mahogany wood. That person has been there longer than how many minutes? Please understand that I'm not asking how long the person has been lying there but for at least how long?

You come upon a dazed person unsteady on their feet with an injury to the face. The blood flow has stopped: the person may have smeared the blood when touching their face but you see that the blood has not changed to a deep brown and still retains a bright red color..approximately how much time has likely elapsed between the injury occurring and your encounter with the injured person?

I know there's no exact time estimate possible without testing but simply for quick field assessments, is anyone willing to hazard an opinion?

In summary: How can I tell the age of a blood stain?

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  • $\begingroup$ It appears you are asking about blood clotting time. You should clarify your question to prevent misunderstandings on what the question is about. $\endgroup$ – March Ho Feb 14 '15 at 19:02
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    $\begingroup$ I don't think it's unclear at all what's be asked; it's not about clotting necessarily. $\endgroup$ – canadianer Feb 14 '15 at 19:06
  • $\begingroup$ If the coloring process is gradual, then the question needs specification as to what the PO regards as the end point $\endgroup$ – AliceD Feb 14 '15 at 20:21
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    $\begingroup$ Good Gravy - thanks so much ...while not the answer I seek, very interesting none-the -less $\endgroup$ – Nikita Bunsen Mar 3 '15 at 23:11
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    $\begingroup$ I edited this... Y'all see what he wants now? $\endgroup$ – L.B. Mar 4 '15 at 1:12
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In comments it been discussed that clotting is partial answer to OP. I would say, that is is somewhat tangential to the original question.

If I may re-phrase, OP asks: how one can estimate age of blood that has been exposed to air? Problem is that you will have to have a starting point to solve that backwards, like having an initial condition for solving differential equation.

One example that can be used is measurement of $pCO_2$ content in blood serum. If it is available, someone already collected this data. In 2 hours $pCO_2$drops 30% and total $CO_2$ drops about 6%.

Moreover, it seems that during hemorrhagic shock RBC shape changes, which means that sampling patient you will be able to approximate time since bleeding started. However, you will need an electron microscope at your disposal...

Finally, and I'll leave it like that, cellular death is a powerful process that leaves a lot of traces. Apoptotic cells have mitochondria bursting (except in RBC there are no mitochondria) and releasing, for example, cytochrome C. In case of bleeding probable cause of cellular death is necrosis, so different biochemical changes will occur. Point is that you can measure time since necrosis/apoptosis started by careful measurement.

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