Creatinine, a humble muscle waste product, is how doctors read the kidneys
Your muscles shed a steady trickle of a waste chemical called creatinine, at a rate set mostly by how much muscle you carry. Because healthy kidneys flush it out almost entirely and unchanged, its level in the blood works as a handy gauge of how well they filter. It is also checked to catch diluted urine drug tests.
Kreas, meaning flesh in ancient Greek, supplies the name. Creatinine is a byproduct of the energy system muscles use for quick bursts. The liver builds creatine from ingredients made in the kidney, the blood carries it to muscles and brain, and there it is loaded with phosphate to become an energy reserve. During that conversion some creatinine forms spontaneously. Men typically produce around 150 to 200 micromoles per kilogram of body weight each day, women somewhat less.
The kidneys remove it mainly by filtering blood through their glomeruli, with a little extra actively secreted by the tubules and almost none reabsorbed. That makes blood creatinine a convenient proxy for the glomerular filtration rate, the core measure of kidney performance. A typical reference range is 0.6 to 1.3 milligrams per decilitre. Formulas combine the reading with details such as age and sex to produce an estimated filtration rate without a 24-hour urine collection.
The gauge has blind spots. Creatinine climbs only after substantial damage to the kidney's filtering units, so it is a late signal rather than an early warning. When kidneys fail badly, tubular secretion makes up a bigger share of clearance, and calculations then overstate how well filtration is going. Readings can also rise for reasons unrelated to kidneys: eating lots of cooked meat, since heat converts creatine into creatinine, taking creatine supplements, intense exercise, dehydration or certain drugs that block secretion. Comparing it with blood urea can hint at problems that originate outside the kidney.
Stranger findings keep surfacing. Laboratory studies suggest creatinine can kill many kinds of bacteria, including antibiotic-resistant strains, while leaving fungi and yeasts alone, a quirk that can help isolate slow-growing fungi. How it does so remains unknown.
Source: Creatinine
