The Marker Changed, Not My Kidneys
One panel flagged my creatinine above range and put my eGFR at 74, which reads as stage 2 kidney disease. The test that settles it came back this morning. Same kidneys, same morning, 47 points apart.
When I ran the extended panel on 3 August, one number came back ugly. Serum creatinine 1.24 mg/dL, above the lab's range of 0.73 to 1.18, with a creatinine-derived eGFR of 74 mL/min/1.73m².
Taken at face value, 74 sits in the band routinely labelled stage 2 chronic kidney disease. One estimate does not establish chronic anything, but the printed number was bad enough to need an answer rather than a shrug.
The answer arrived this morning, and it is a 47-point swing.
Same kidneys, same blood, three numbers
The reason I ordered cystatin C in the first place was written down before the needle went in: I take creatine daily, and I had put on muscle across three months of training six days a week. Both raise creatinine production without touching filtration. So repeating creatinine would only have repeated the ambiguity. Cystatin C changes the input variable.
It came back at 0.68 mg/L, reference 0.62 to 1.10, by nephelometry.
The creatinine-only calculation reproduces the lab's own printed eGFR exactly, which is the check that matters — it means the 47-point gap is not me using a different formula than they did. Same equations, same blood, same morning. The creatinine says 74. The cystatin C says 121.
Cystatin C landed 0.06 above the floor of its interval, roughly 13% of the way through it, and for cystatin C lower means better filtration. The combined estimate at 101 still carries some of the distorted creatinine signal, which is why it sits below the cystatin-only number — and it is still comfortably normal.
These are estimates, not a measured filtration rate. I am not claiming my kidneys clear exactly 121. The useful part is simpler: the marker that is blind to my two known confounders returned a result incompatible with kidney disease.
Why creatine does this
Creatinine is the spontaneous, non-enzymatic breakdown product of creatine and creatine phosphate in muscle. Supplementing creatine enlarges the pool that degrades, so more creatinine is produced per day. The kidneys are not failing to clear it. They are clearing the same fraction of a bigger input.
Creatine typically adds 0.1 to 0.3 mg/dL to serum creatinine with no change in filtration. Mine rose 0.09 from the previous panel — slightly below even that.
Muscle mass does the same thing by a different route. Daily creatinine production runs roughly proportional to total muscle mass, because more muscle means more creatine and creatine phosphate available to break down. The equations adjust for age and sex, but they cannot see your body composition. They assume an average relationship between the variables in the formula and daily creatinine production — and a heavily muscled person breaks that assumption in the direction that looks like disease.
A creatinine-based eGFR cannot see why creatinine went up. It sees a concentration and converts it into a filtration rate, on the assumption that production is average. When production rises, the maths reads the rise as lost clearance.
Both of my confounders pushed the same way at once: creatine enlarged the pool, and added muscle enlarged the tissue producing from it. A mathematically correct calculation produced a clinically misleading number.
Why cystatin C answers instead
Cystatin C is a low-molecular-weight protein produced at a near-constant rate by all nucleated cells, freely filtered by the glomerulus, and largely independent of muscle mass and creatine intake. That independence is the entire reason to order it.
It is not confounder-free either — thyroid dysfunction, corticosteroids, obesity and inflammation all move it. But none of those confounders is muscle or creatine, which are precisely the two that were already on the record before my blood was drawn.
The part that generalises
If you lift seriously, carry more muscle than average and supplement creatine, a creatinine-based eGFR is structurally the wrong instrument for your body. Sooner or later it will hand you a frightening number, and the number will be arithmetically correct and clinically wrong.
The fix cost one extra line on a lab order. It converted a hand-wavy argument about training and supplements into a quantified comparison: 74, 121, 101. My kidneys did not change between those three numbers. Only the marker did.
So the operating rule from here: while I am on creatine and carrying this much muscle, renal questions get answered with cystatin C, not creatinine — ordered alongside it on every panel, not instead of it. The pair is more informative than either alone, and the gap between them is itself a measurement.
Five results still out
The panel is not closed. Five ordered markers are still with the lab, and two of them are the ones I actually want.
Those close the panel. The kidney question is already closed: creatinine raised the alarm, cystatin C tested the alarm, and the alarm did not survive the better measurement.
Twenty-One Markers and an ECG — the panel this came from, including why cystatin C was on the order.
The Deficit Beat the Peptides — the headline finding from the same draw.
These Are the Final Results. Nooot. — the panel before it, where the creatinine first went above range.
MONSTER 2.0 — The Blueprint and The Kitchen — the training and the food that built the muscle doing the confounding.
