Cystine

If you have ever passed a kidney stone, the question that matters most is whether another one is forming right now. For people with cystinuria, a 24-hour urine cystine test answers that question more directly than almost any other lab. It captures a full day of what your kidneys are dumping into your urine, which is the precise pool of material that crystallizes into stones.

This is also one of the few stone tests where the result drives a real treatment plan. The numbers tell you whether your fluid intake is working, whether your urine is dissolving cystine the way it should, and whether the medications meant to keep stones away are actually doing their job.

What This Test Actually Measures

Cystine is a small molecule made when two cysteine amino acids link together. It is not a protein, hormone, or enzyme. In healthy kidneys, cystine that gets filtered into urine is mostly pulled back into the bloodstream by transporters in the kidney's filtering tubes. In cystinuria, an inherited condition, those transporters don't work, so cystine piles up in urine. Because cystine doesn't dissolve well in water, it can crystallize and form stones.

The 24-hour collection captures the entire day's urine output rather than a single snapshot. That matters because cystine concentration shifts throughout the day depending on what you eat, how much you drink, and your urine's acidity. A full-day collection averages out those swings and gives a number that reflects your real daily exposure.

Why It Matters: Cystine Stone Risk

Cystine stones are a lifelong problem. They tend to start in childhood or young adulthood, recur often, and require repeated procedures to remove. The 24-hour cystine test exists to catch the conditions that produce them before the next stone forms.

Among adults with cystinuria followed for an average of 11.6 years, the patients whose stone formation was arrested had significantly higher daily urine volume than those who kept making stones (around 3,151 mL per day versus 2,446 mL per day). High urine volume dilutes cystine and lowers the chance it crystallizes. In a separate study of cystinuric patients, those whose first stone happened later in life had higher urine volumes and lower cystine concentrations than those who started young, despite excreting similar total cystine.

What this means for you: the goal is not just to lower your cystine excretion. It is to keep cystine dissolved. Volume, urine acidity, and binding medications all act on the same problem from different angles, and a 24-hour collection is the only test that lets you see whether your strategy is actually working.

Cystine Capacity and Stone Activity

A specialized version of this test, called cystine capacity, measures how much extra cystine your urine could hold in solution before it crystallizes. The widely cited target was a capacity above 150 mg/L, but a study of 48 cystinuric patients followed for nearly six years found that this threshold caught only about 8% of stone-quiescent periods. Lowering the cutoff to 90 mg/L improved detection to about 25% while keeping specificity at roughly 91%.

In the same study, mean cystine capacity during quiescent periods was higher than during active stone formation, and capacity correlated strongly with cystine supersaturation and concentration. What this means for you: the older 150 mg/L target may be too lenient, and a stricter goal is more realistic for keeping stones away.

Beyond Stones: What Cystine Reflects Elsewhere

Cystine is also part of your body's antioxidant system, the network that neutralizes the chemical wear and tear of normal metabolism. The 24-hour urine measurement is mainly used for stone risk, but blood-based cystine measurements (a different test) have been linked to cardiovascular outcomes in coronary artery disease and to inflammatory bowel disease. Those findings are not direct evidence about your urine result, and they should not be interpreted as such, but they do help explain why cystine matters biologically.

Reference Ranges

There are no broad population reference ranges for 24-hour urinary cystine. The thresholds in the literature come from cystinuria research and apply mainly to patients with the condition. Healthy adults typically excrete under 30 mg of cystine per day, while people with the homozygous form of cystinuria often excrete more than 400 mg per day. Treatment thresholds are based on cystine capacity rather than total excretion.

These ranges come from cystinuria cohorts and a single proprietary capacity assay (Litholink). They are illustrative orientation, not universal targets. Different labs may report different units or use different cutpoints.

Compare your results within the same lab over time for the most meaningful trend. Capacity assays in particular are not interchangeable across labs.

Tracking Your Trend

A single 24-hour cystine result is a starting point, not a verdict. Within-person variability is significant: the same person, on the same medications, can produce different numbers from one collection to the next depending on protein intake, fluid intake, and how completely they captured the day. In a study of serial 24-hour urine collections from active stone formers, repeating the test at six-month or longer intervals significantly improved stone-risk parameters, suggesting that ongoing monitoring drives better outcomes than a single measurement.

Practically, this means: get a baseline. If you start a thiol medication, increase fluids, or shift your diet, retest in roughly three to six months to confirm the change is reflected in your urine. After that, at least annual collections are reasonable for anyone with active stone disease, and more often if you are still adjusting therapy. Two collections at the start are better than one, because clinical decisions can change in nearly half of patients when a second collection is added.

Decision Pathway for Abnormal Results

If your 24-hour cystine is high or your capacity is low, the pathway is concrete. First, look at urine volume. If you are below 3 liters per day, that is the first lever to pull, because volume is the only factor consistently linked to arrested stone formation in long-term studies. Second, check urine pH. Most management protocols target a pH of around 7.5, achieved with potassium citrate or potassium bicarbonate. Third, if volume and alkalinization are optimized and capacity is still low, the conversation moves to thiol medications such as tiopronin or D-penicillamine, which bind cystine into a more soluble form.

Companion tests worth ordering alongside this one include a full 24-hour urine stone panel (volume, pH, sodium, citrate, calcium, oxalate, uric acid), basic kidney function (eGFR, creatinine), and, in unclear cases, genetic testing for SLC3A1 and SLC7A9 variants to confirm cystinuria. A urologist or nephrologist with stone disease experience is the right specialist if you have recurrent stones or your numbers are not improving with therapy.

When Results Can Be Misleading

A single 24-hour collection can mislead in several specific ways:

• Incomplete collection: missing even a few hours of urine, or forgetting to capture the morning void after starting the clock, will produce a falsely low total. Confirming completeness with creatinine output is standard practice.
• Recent dietary swings: higher protein intake raises cystine excretion. In one cohort, an extra 10 grams of daily protein corresponded to about 164 micromoles more cystine per day. A single high-protein week before testing can shift your number.
• Thiol medication interference: if you are on tiopronin or D-penicillamine, standard cystine assays may measure both free cystine and drug-bound cystine, depending on the lab. The HPLC-MS/MS method (a more precise lab technique) reports more accurate values for patients on these drugs.
• Day-night variation: cystine supersaturation can spike at night when urine is most concentrated, and a 24-hour total may smooth over the riskiest hours. Some clinicians order separate day and night collections in difficult cases.

Who Should Test

24-hour cystine testing is the standard of care for anyone with confirmed cystinuria, recurrent kidney stones of unclear composition, or a family history of cystine stones. It is not a general-population screening test. Despite clear guidelines, only about 7 to 8% of high-risk recurrent stone formers in a large US claims database received a 24-hour urine collection within six months of diagnosis, which is a substantial gap in care for people who would clearly benefit.