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The topic of AKI and CKD is fairly extensive. Below, we present a primer on the general diagnosis and approach to these conditions along with relevant calculators.

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• Before we can talk about AKI, we have to discuss normal renal function (GFR). 

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• We use creatinine as an indirect marker of kidney function ( estimated, or eGFR) since it is produced at a constant rate, freely filtered and not secreted nor reabsorbed in sufficient quantities to have an impact. 

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• Creatinine is influenced by muscle mass and fluid status and begins to rise only ~ 48+ hours from the injury. It is therefore only a reliable marker of eGFR in the steady state and must be used cautiously in specific populations (immobile patients, etc.). It is impossible to estimate eGFR in a patient who's creatinine is changing. We may therefore provide guidance to dose medications even based on eGFR < 15 ml/min/1.73m2 if creatinine is changing rapidly. What we CAN say, is where the creatinine increases by a specific threshold (see below), that there is significant kidney injury. 

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• In the neonate, eGFR changes rapidly (see table below). This is why we use different dosing for renally-cleared medications- the creatinine doesn't tell the whole story! Of note, neonatal AKI is somewhat different and not fully addressed in this blog entry, though the principles are similar.

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• Cystatin C is produced by all nucleated cells at a constant rate. It is increasingly being used as more "reliable" marker of kidney function in the steady state and in AKI. We now use a formula to combine both calculations to provide a more accurate measurement of eGFR. At this time, Cystatin C as a measure of kidney function is only used in children > 2 years 

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• Normal kidney function, AKA clearance, for a child above 2 years old through adulthood is 100-120ml/min/1.73m2.​

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Classification and Causes of AKI

• Fundamentally, we typically classify AKI by location of injury: Pre-renal, Intrinsic and Post-renal. Understanding the type of AKI affects management. For example, pre-renal AKI is typically a volume/perfusion problem, and is therefore fluid responsive. Intrinsic AKI is damage to the kidney tubules, and is typically not fluid responsive, in which case, continuing fluid resuscitation in someone who may not be making much urine can lead to fluid overload and other complications, (See "Fluids and Electrolytes" for management tips), but in short, if there is decreased uop, you may consider insensibles + uop replacement.

• You may recall the BUN:Cr ratio of >20:1 indicates pre-renal, <20:1 may be intrinsic injury. This value is often reported by commercial labs, however this is misleading if BUN and creatinine are normal for age/individual patient since BUN does not change a whole lot in the life course. For example  a toddler could have BUN 10, creatinine of 0.4 = BUN/Cr of 25:1 and this is completely normal. However, in AKI this is useful as the kidney increases BUN reabsorption in the face of volume depletion but creatinine does not increase as much, leading to the increased ratio and high specific gravity as the tubules maximally reabsorb water. In ATN, the tubules are damaged and reabsorptive capacity is limited, therefore serum BUN is lower than expected for the creatinine and specific gravity is low. (See "UA interpretation" for more)​

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• To differentiate, we often obtain FeNa measurement. This will essentially determine if the tubules are intact. i.e., if it is a volume depletion problem with intact tubules, the kidney will reabsorb as much as Na+ as possible, thereby leading to a low excretion of sodium->low FeNa. However, if the tubules are damaged, sodium cannot be reabsorbed-> elevated FeNa. (see "FeNA calculator" below)

• However, FeNa is not accurate if the patient received fluids ( see "Fluids and Electrolytes" section for daily Na requirements)  or diuretics, as the urinary sodium has been manipulated.

• In which case, FeUrea, which is the same formula as FeNa except replacing sodium with urea, is preferred since the result is not affected by fluids/diuretics. Never fear, there is a calculator below!