Not Every Drop of a Person’s Blood Is the Same, a Study Says
nytimes.com
nytimes.com
Not only does it seem like a very fundamental question to have already asked...it doesn't even seem like a very difficult study to do. It's not substantially longitudinal over time -- for every subject, you take several pinpricks, and run the tests. Or logistically difficult to manage.
So I get why it's news, in terms of Theranos and what not...but this has to have been something that was studied many times over many decades. Or is the NYT misinterpreting/signifying the significance, i.e. the Rice scientists found a previously undetectable kind of difference, but which is, yes, technically shows that blood drops are different?
The level of understanding required to build a thing is very different from the level of understanding required to patch a somewhat broken system.
- What are the units on blood pressure? 120 of what?
- Would it make a difference if I held my arm up while the blood pressure was taken?
Meanwhile, over here in the dev world, our passionate mantra is "we shouldn't be judged poorly for 'everyone knows' shit we can just google"...
First of all, in the case from my past, the machine labeled it -- "mmHg". They could have read it off, had they expected things to have units that matter.
Second, it's part of the conceptual understanding of what blood pressure means -- that e.g. it's not some normalized percentage, that it indicates how much the fluid inside is pressing against the outside, that it's relative to atmospheric pressure, that we live in and expect a pressurized environment.
Third -- I mean, you read it out every day, wouldn't you ever wonder what it means? Imagine a Java dev that doesn't know what System is, only that it's the beginning of the stdout print statements they use.
Someone who merely knows that "180 bad, 120 good" has a disconnected understanding, like the expert who can literally do nothing more than plug numbers into an equation but not know what the numbers mean or whether you're measuring them correctly to be compatible with it. It's not enough.
This study is at the "microtiter" scale, which is the scale that companies such as theranos are trying to take advantage of.
Microfluidics is largely an industrial field rather than academic.
>Not only does it seem like a very fundamental question to have already asked...it doesn't even seem like a very difficult study to do.
Yeah, well, that's the difference between Silicon Valley and academia.
SV doesn't want to hear about results that invalidate their business model.
I promise you that academics not wanting to hear results that invalidate their models is the rule, not the exception.
This essentially equivalent to a sampling error problem. A large venous sample (10 mL) is enough to get a pretty good average. Microliters of blood from any given location in the body are likely to be different from microliters somewhere else in the body. I seriously doubt anyone in a clinical lab would be surprised by these results.
This study is about novel tests based on minute volumes of blood. These are not routine in medical practice.
If noise is too high for a single drop, a venous draw is a much larger volume and theoretically equivalent to sampling many drops of blood—it's the physical equivalent to averaging samples to increase the SNR.
The authors note[1] that averaging may not be enough though, and that there may be an interesting difference inherent to fingerprick blood (possibly caused by their collection method):
"Our data also suggest that collecting and analyzing more fingerprick blood does not necessarily bring the measured value closer to those of the donor’s venous blood (Figures 1D and 2D). For example, donor B’s hemoglobin and WBC concentration were similar for venous blood and fingerprick in drop 1 but became less concordant with additional drops, while donor C’s fingerprick measures came closer to the venous measures with additional drops. These data may represent true differences between fingerprick and venous blood, or they may be the result of errors in collection (such as leaving the tourniquet on for too long during a venous draw). Further research is needed to determine how common these patterns are."
1. http://ajcp.oxfordjournals.org/content/ajcpath/144/6/885.ful...
I wonder if like how red blood cells would settle to the bottom in a test tube one might expect that there would be differing concentrations of blood constituents in venus blood in the extremities depending on the elevation, temperature, perfusion, etc... of the extremity?
[1]http://www.hindawi.com/journals/isrn/2012/508649/ [2]http://www.ncbi.nlm.nih.gov/pubmed/23294266 [3]http://journals.plos.org/plosone/article?id=10.1371/journal....
> Morris et al7 believe the higher variability of capillary blood compared with venous blood is due to the presence of extracellular fluid in capillary samples. In clinical practice, milking of the finger by insufficiently trained health care workers may result in even greater drop-to-drop variability than shown here.
http://ajcp.oxfordjournals.org/content/144/6/885.full
It's a very interesting problem. They didn't even see all the donors end up with the same average from drops as from a venous draw.
> Our data also suggest that collecting and analyzing more fingerprick blood does not necessarily bring the measured value closer to those of the donor’s venous blood (Figures 1D and 2D). For example, donor B’s hemoglobin and WBC concentration were similar for venous blood and fingerprick in drop 1 but became less concordant with additional drops, while donor C’s fingerprick measures came closer to the venous measures with additional drops. These data may represent true differences between fingerprick and venous blood, or they may be the result of errors in collection (such as leaving the tourniquet on for too long during a venous draw). Further research is needed to determine how common these patterns are.
Also:
> Morris et al7 believe the higher variability of capillary blood compared with venous blood is due to the presence of extracellular fluid in capillary samples. In clinical practice, milking of the finger by insufficiently trained health care workers may result in even greater drop-to-drop variability than shown here.
This may present more of a risk for at-home testing if you need to make sure that the person hasn't squeezed their finger at all.
Quite an interesting set of problems that I'd not really considered before.
Full article: http://ajcp.oxfordjournals.org/content/144/6/885.full
Also, for blood glucose, which diabetics need to monitor closely, there is some buzz now around spectrographic (IR/UV) techniques, meaning you don't even have to puncture the skin. That would be huge. Diabetics actually complain more about fingerprick tests for glucose than about insulin injections, which sounds very counter-intuitive to non-diabetics.
What glucose and CRP have in common is that they are small. TFA talks about tests for white blood cells, platelets or HIV, which are much larger. If you compare white blood cells to glucose, they have three orders of magnitude larger radius, so nine orders of magnitude larger volume.
That's like the difference in volume between a raindrop and a blue whale. No wonder different mechanisms may apply.
It would be huge, yes. But it has been "five years away from being available" for about 25 years. Glucose has a much smaller signal than the Hb / HbO2 signal used for pulse oximetry.
Diabetics actually complain more about fingerprick tests for glucose than about insulin injections, which sounds very counter-intuitive to non-diabetics.
Most non-diabetics think that "injecting insulin" involves hitting a vein. That would be far more painful. Instead, we inject into subcutaneous tissue; and the needles we use are absolutely tiny. It's worth noting that it's very unusual to get any bleeding from an injection site.
For all intents and purposes, your DNA is the same in every cell. We know this because we can pool DNA from multiple cells and sequence them. We see orders of magnitude more variation from sequencing error than true intra-isolate nucleotide variation.
Yes epigenetics exists, and it influences our development and habits, but it's not going to change the content of your DNA.