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| Assay principle | In the Jaffe reaction, creatinine is treated with an alkaline picrate solution to yield a bright orange-red complex. Diluted samples are added to a microplate, and alkaline picrate reagent is added and incubated at room temperature for 30 minutes. Intensity of the color at 490 nm corresponds to the concentration of creatinine in the sample. Unknown samples are compared to the standard curve. |
| Sample Type | Urine samples |
| Assay Time | 30 min |
| Assay Range | 0.3-20 mg/dL |
| Sensitivity | 0.1 mg/dL |
| Specificity | No cross reactivity with relevant targets |
| Sample Level | 11-250 mg/dL |
| Dilution Factor | 1/20 |
| Background | Creatinine is a metabolite of phospho-Creatine (p-Creatine), a molecule used as a store for high-energy phosphate that can be utilized by tissues for the production of ATP (1). The source of Creatine may vary between species. Dietary intake may be accompanied by de novo synthesis from the amino acids arginine, glycine, and methionine. This occurs in stepwise fashion primarily in the kidneys and liver, although other organ systems may be involved and species-specific differences may exist (2). The vast majority of Creatine (> 90%) is found in muscle, as well as other tissues including heart, brain, photo-receptors, and testes (2-7). Creatine and p-Creatine are converted non-enzymatically to the metabolite Creatinine (Figure 1), which diffuses into the blood and is excreted by the kidneys. In vivo, this conversion appears to be irreversible and in vitro it is favored by higher temperatures and lower pH (2). Some Creatinine may form via p-Creatinine as well (8). Under normal conditions, Creatinine's formation occurs at a rate that is relatively constant. For instance, in humans approximately 2% of the Creatine/p-Creatine pool is converted to Creatinine daily (2). This predictability makes Creatinine a useful tool for normalizing the levels of other molecules found in urine (9, 10). In addition, altered Creatinine levels can be used as an indicator of kidney dysfunction, or may be associated with other conditions that result in decreased renal blood flow (11). Some examples include diabetes and cardiovascular disease (12, 13). |