How is low extra cellular fluid levels diagnosed in alkalosis?can it be present despite electrolyte blood test showing normal electrolyte levels?

There is a test using Evans Blue to measure extracellular fluid volume, but we physicians don't use it. Instead we know that depletion of extracellular volume leads to depletion & contraction of circulating & intracellular volume anyway. Serum sodium & chloride can be high or low and BUN generally high in this setting.

Yes electrolyte levels are not helpful. Will look dry and be thirsty. Veins will be flat, May get dizzy when standing. Hc may be high. Urine will be darker. Has nothing to do with alkalosis usually. You may want to invest $20 in uptodate.com to learn more.