Creatinine Ratio Calculator

The urine/serum creatinine ratio compares the concentration of creatinine in urine to that in serum. It is used in the workup of acute kidney injury (AKI) to help differentiate prerenal causes from intrinsic renal causes. A high ratio suggests prerenal azotemia.

The ratio is calculated as Urine Creatinine (mg/dL) divided by Serum Creatinine (mg/dL). For example, if urine Cr = 100 mg/dL and serum Cr = 1.0 mg/dL, the ratio is 100.

A ratio > 40 suggests prerenal azotemia. A ratio < 20 is more consistent with intrinsic renal disease (ATN). Values between 20–40 are indeterminate and require clinical correlation.

The fractional excretion of sodium (FeNa) is the percentage of filtered sodium that is excreted in urine. It is a key test in differentiating prerenal AKI from intrinsic AKI (ATN). Formula: FeNa% = (Urine Na / Serum Na) / (Urine Cr / Serum Cr) × 100.

FeNa < 1% suggests prerenal azotemia (sodium-retaining state). FeNa 1–2% is indeterminate. FeNa > 2% suggests intrinsic renal disease (ATN). Note: FeNa can be < 1% in some intrinsic diseases like acute glomerulonephritis and contrast nephropathy.

Low FeNa (< 1%) is seen in prerenal conditions: volume depletion (hemorrhage, dehydration), heart failure, cirrhosis, and early acute glomerulonephritis. The kidneys are retaining sodium appropriately.

High FeNa (> 2%) is seen in acute tubular necrosis (ATN), post-renal obstruction recovery, and intrinsic renal disease. The tubules have lost the ability to reabsorb sodium.

FeNa can be misleading in patients on diuretics (which increase Na excretion), in CKD (baseline FeNa is higher), in acute glomerulonephritis (FeNa may be low despite intrinsic disease), and in early sepsis (variable FeNa).

Fractional excretion of urea (FeUrea) is an alternative to FeNa that is less affected by diuretics. FeUrea% = (Urine Urea / Serum Urea) / (Urine Cr / Serum Cr) × 100. FeUrea < 35% suggests prerenal; > 50% suggests intrinsic AKI.

FeUrea is preferred in patients receiving diuretics, as loop diuretics directly increase sodium excretion and falsely elevate FeNa. FeUrea is also useful in differentiating of prerenal azotemia from ATN in ICU settings.

The protein/creatinine ratio is a spot urine test that estimates 24-hour protein excretion. PCR = Urine Protein (mg/dL) / Urine Creatinine (mg/dL). A PCR < 0.15 (or < 150 mg/g) is normal.

PCR on a spot urine sample correlates well with 24-hour urine protein excretion. A PCR of 1.0 corresponds to approximately 1 gram of protein per day. It is more convenient than 24-hour urine collection.

Normal PCR is less than 150 mg/g (or < 0.15). Values between 150–500 mg/g indicate mild proteinuria. Values > 500 mg/g indicate significant proteinuria requiring further workup.

Proteinuria can be transient (fever, exercise, stress, urinary tract infection), orthostatic (benign, occurs when upright), or persistent (glomerular diseases like diabetic nephropathy, glomerulonephritis, hypertensive nephropathy, and multiple myeloma).

Nephrotic-range proteinuria is defined as PCR > 3.5 (or > 3500 mg/g), or 24-hour protein > 3.5 g. It is seen in nephrotic syndrome, which includes minimal change disease, focal segmental glomerulosclerosis (FSGS), membranous nephropathy, and diabetic nephropathy.

Creatinine clearance (CrCl) uses both the urine/serum Cr ratio and urine volume to estimate GFR. CrCl = (Urine Cr × Volume) / (Serum Cr × Time). The urine/serum Cr ratio alone does not account for urine volume.

AKI is a sudden decline in kidney function over hours to days, characterized by rising serum creatinine and/or decreased urine output. Causes are classified as prerenal (volume depletion, heart failure), intrinsic (ATN, glomerulonephritis, interstitial nephritis), or postrenal (obstruction).

Prerenal azotemia is a reversible decrease in GFR due to reduced renal blood flow. It is characterized by FeNa < 1%, urine/serum Cr ratio > 40, elevated BUN/Cr ratio > 20, and FeUrea < 35%. It resolves with volume repletion.

ATN is intrinsic renal injury due to ischemic or nephrotoxic damage to tubular cells. It is characterized by FeNa > 2%, urine/serum Cr ratio < 20, muddy brown granular casts on urinalysis, and a slower recovery course over days to weeks.

A BUN/Cr ratio > 20 suggests prerenal azotemia (increased urea reabsorption). A ratio of 10–15 is typical in intrinsic AKI. However, the ratio can be affected by GI bleeding, steroid use, and dietary protein intake.

Muddy brown granular casts are a classic urinalysis finding in ATN. They represent sloughed tubular epithelial cells and cellular debris. Their presence strongly supports a diagnosis of intrinsic AKI due to ATN.

CIN is AKI occurring within 48–72 hours of intravenous iodinated contrast administration. FeNa is typically low (< 1%) despite intrinsic injury. Risk factors include CKD, diabetes, heart failure, and high contrast volume.

Loop diuretics block the Na-K-2Cl cotransporter in the thick ascending limb, increasing sodium excretion and falsely elevating FeNa. This may lead to a misdiagnosis of intrinsic AKI when prerenal physiology is actually present.

Urine osmolality helps differentiate prerenal (> 500 mOsm/kg, concentrated urine) from intrinsic (< 350 mOsm/kg, isosthenuric) AKI. However, it can be influenced by diuretics, glucose, and mannitol.

HRS is a functional renal failure occurring in advanced liver disease. FeNa is very low (< 1%) despite severe AKI, reflecting intense renal vasoconstriction. It requires liver transplantation for definitive treatment.

Proteinuria in CKD is staged by ACR (albumin-to-creatinine ratio): A1 < 30 mg/g (normal), A2 30–300 mg/g (moderately increased), A3 > 300 mg/g (severely increased). PCR is used similarly but includes all proteins.

The urine albumin-to-creatinine ratio (ACR) specifically measures albumin excretion. It is more sensitive than PCR for detecting early diabetic nephropathy. Normal ACR is < 30 mg/g.

Annual screening for proteinuria (ACR or PCR) is recommended for all patients with type 1 diabetes (starting 5 years after diagnosis) and type 2 diabetes (at diagnosis). More frequent monitoring is indicated if abnormal.

KDOQI classifies CKD by GFR category (G1–G5) and albuminuria category (A1–A3). For example, G3aA2 indicates moderate GFR reduction (45–59) with moderately increased albuminuria. This guides prognosis and management.

Nephrology consultation is indicated for rapidly rising creatinine, AKI requiring dialysis, nephrotic-range proteinuria, unexplained hematuria with proteinuria, suspected glomerulonephritis, CKD stage 4 or 5, and electrolyte abnormalities refractory to management.