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My understanding is that the test works by pinching the back of your own hand for two seconds, then letting go. If the pinch "immediately" returns flat you are not dehydrated but if it doesn't, you are dehydrated.
How long is "immediately"?
Do I need to be sitting and relaxed or anything?
According to Healthline the skin sould bounce back within a second or two. The pertinent passage is:
When you pinch the skin on your arm, for example, it should spring back into place with [sic] a second or two.
The article also mentions that this should be done on your arm or abdomen. MedlinePlus says the same.
This is called skin turgor. It is not considered reliable for those over sixty-five. It can take over twenty seconds for the skin of elderly individuals to return to normal, because we lose elasticity as we age.
In general, this finding is not 100% reliable on its own, and when using this to determine volume status it should be to corroborate other findings, not as diagnostic on its own. Correlate with history (decreased fluid intake, decreased urination), symptoms (dry mouth, oliguria, lightheadedness, palpitations), vital signs (tachycardia, hypotension), moistness of mucous membranes, labs (lactate, anion gap), etc depending on the case.
