true because the episode of AKI began prior to medical attention, and thus the day 1 SCr level was already increased. If creatinine measurements had available with 48 hours prior to day 1 and if this level had been at baseline (1.0 mg/dl [88.4 mmol/l]), it would have been possible to diagnose AKI on day 1 using the second criterion. Cases F-H do not have a baseline measurement of SCr available. Elevated SCr (reduced eGFR) on day 1 of the hospitalization is consistent with either CKD or AKD without AKI. In Case F, baseline SCr can be inferred to be below the day 1 value because of the subsequent clinical course; thus, we can infer the patient has had an episode of AKI. In case G, AKI can be diagnosed by application of criterion 2, but the patient may have underlying CKD. Case H does not fulfill the definition for AKI based on either criteria, and has either CKD or AKD without AKI. The example of Case A raises several important issues. First, frequent monitoring of SCr in patients at increased risk of AKI will significantly improve diagnostic time and accuracy. If Case A had not presented to medical attention (or if SCr had not been checked) until day 7, the case of AKI would likely have been missed. Frequent measurement of SCr in high-risk patients, or in patients in which AKI is suspected, is therefore encouraged—see Chapter 2.3. The second issue highlighted by Case A is the importance of baseline SCr measurements. Had no baseline been available it would still have been possible to diagnose AKI on day 3 (by either using criterion 2 or by using criterion 1 and accepting the baseline SCr as 1.3); however, not only would this have resulted in a http://www.kidney-international.org chapter 2.4 & 2012 KDIGO Table 7 | AKI diagnosis Serum creatinine mg/dl (lmol/l) Diagnosis AKI? Criterion 1 Criterion 2 Case Baseline Day 1 Day 2 Day 3 Day 7No 28 Kidney International Supplements (2012) 2, 19–36 delay in diagnosis, it would have resulted in a delay in staging (see Table 7). On day 7, it can be inferred that the patient’s baseline was no higher than 1.0 mg/dl (88 mmol/l) and thus correct staging of Case A as Stage 2 (two-fold increase from the reference SCr, see below and Table 7) on day 3 could have been determined in retrospect. However, if a baseline SCr was available to use as the reference, the correct stage could be determined on day 3. Case B illustrates why criterion 2 can detect cases of AKI missed by criterion 1. It also clarifies why these cases are unusual. Had the SCr increased to 1.5 mg/dl (132.6 mmol/l) as opposed to peaking at 1.4 mg/dl (123.8 mmol/l), it would have been picked up by criterion 1 as well. By contrast Cases C, D, and even F illustrate how criterion 2 may miss cases identified by criterion 1. Note that Case F can only be diagnosed by inference. By day 7, it can be inferred that the baseline was no higher than 1.0 mg/dl (88 mmol/l) and thus it can be determined that the patient presented with AKI. However, if the baseline SCr could be estimated it would be possible to make this inference as early as day 1. Estimating baseline SCr 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 Table 8 | Overview of the approaches to determine baseline SCr in the application of RIFLE classification in previous studies Study No. of pts analyzed Multi-/ single-center Criteria used Method to determine baseline SCr % recorded % estimated Bagshaw25 120123 multi cr+uo estimated by MDRD formula 0 100 Ostermann30 41972 multi cr estimated by MDRD formula 0 100 Uchino5 20126 single cr retrieved from hospital database, or estimated by MDRD formula N/A N/A Bell54 8152 single cr+uo retrieved from hospital database, or estimated by MDRD formula N/A N/A Hoste2 5383 single cr+uo estimated by MDRD formula, or admission creatinine value, whatever was lower N/A N/A Ali31 5321 multi cr retrieved from hospital