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Many patients will present with AKI without a reliable baseline SCr on record. Baseline SCr can be estimated using the Modification of Diet in Renal Disease (MDRD) Study equation assuming that baseline eGFR is 75 ml/min per 1.73 m2 (Table 9).22 This approach has been used in many, but not all, studies of AKI epidemiology using RIFLE2,5,25,30–32,54–63 (see Table 8) and has recently been validated.64 Hence, most current data concerning AKI defined by RIFLE criteria are based on estimated baseline SCr for a large proportion of patients.
Table 9 shows the range of estimated SCr obtained by back-calculation for various age, sex, and race categories. When the baseline SCr is unknown, an estimated SCr can be used provided there is no evidence of CKD (see Appendix B). Fortunately, when there is a history of CKD, a baseline SCr is usually available. Unfortunately, many cases of CKD are not identified, and thus estimating the baseline SCr may risk labeling a patient with AKI when in reality the diagnosis was unidentified CKD. As discussed further in Appendix B, it is essential to evaluate a patient with presumed AKI for
| Serum creatinine mg/dl (µmol/l) | Reference creatinine | Max AKI stage | |||||
|---|---|---|---|---|---|---|---|
| Case | Baseline | Day 1 | Day 2 | Day 3 | Day 7 | ||
| A | 1.0 (88) | 1.3 (115) | 1.5 (133) | 2.0 (177) | 1.0 (88) | 1.0 (88) | 2 |
| B | 1.0 (88) | 1.1 (97) | 1.2 (106) | 1.4 (124) | 1.0 (88) | 1.0 (88) | 1 |
| C | 0.4 (35) | 0.5 (44) | 0.6 (53) | 0.7 (62) | 0.4 (35) | 0.4 (35) | 1 |
| D | 1.0 (88) | 1.1 (97) | 1.2 (106) | 1.3 (115) | 1.5 (133) | 1.0 (88) | 1 |
| E | 1.0 (88) | 1.3 (115) | 1.5 (133) | 1.8 (159) | 2.2 (195) | 1.0 (88) | 2 |
| F | ? | 3.0 (265) | 2.6 (230) | 2.2 (195) | 1.0 (88) | 1.0 (88) | 3 |
| G | ? | 1.8 (159) | 2.0 (177) | 2.2 (195) | 1.6 (141) | ? | ≥1 |
| H | ? | 3.0 (265) | 3.1 (274) | 3.0 (265) | 2.9 (256) | ? | ? |
AKI, acute kidney injury.
presence of CKD. Furthermore, CKD and AKI may coexist. By using all available clinical data (laboratory, imaging, history, and physical exam) it should be possible to arrive at both an accurate diagnosis as well as an accurate estimate of baseline SCr. Importantly, excluding some cases of hemodilution secondary to massive fluid resuscitation (discussed below), the lowest SCr obtained during a hospitalization is usually equal to or greater than the baseline. This SCr should be used to diagnose (and stage) AKI. For example, if no baseline SCr was available in Case A, diagnosis of AKI could be made using the MDRD estimated SCr (Table 9). If Case A were a 70-year-old white female with no evidence or history of CKD, the baseline SCr would be 0.8 mg/dl (71 µmol/l) and a diagnosis of AKI would be possible even on day 1 (criterion 1, ≥ 50% increase from baseline). However, if the patient was a 20-year-old black male, his baseline SCr would be estimated at 1.5 mg/dl (133 µmol/l). Since his admission SCr is lower, this is assumed to be the baseline SCr until day 7 when he returns to his true baseline, and this value can be taken as the baseline. These dynamic changes in interpretation are not seen in epidemiologic studies, which are conducted when all the data are present, but are common in clinical medicine. Note that the only way to diagnose AKI (by SCr criteria) in Case H is to use an estimated SCr.
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