40 year male serum creatinine 2.39 mg/dl serum cystatin c 1.13 mg/L calculate EGFR
Reading File
The CKD-EPI 2021 (creatinine-cystatin C) equation is the most accurate formula when both creatinine and cystatin C are available. Let me calculate it directly.
eGFR Calculation
Patient: Male, 40 years old
Serum Creatinine: 2.39 mg/dL
Serum Cystatin C: 1.13 mg/L
Formula Used: CKD-EPI Creatinine–Cystatin C 2021
This is the recommended equation when both markers are available — it is more accurate than using either alone.
eGFR = 135 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^(−0.601) × min(Scys/0.8, 1)^(−0.375) × max(Scys/0.8, 1)^(−0.711) × 0.9938^Age
For males: κ = 0.9, α = −0.302
Step-by-Step Calculation
Creatinine component:
- Scr/κ = 2.39 / 0.9 = 2.656 → this is > 1, so:
- min(2.656, 1)^(−0.302) = 1^(−0.302) = 1.000
- max(2.656, 1)^(−0.601) = 2.656^(−0.601) = e^(−0.601 × ln(2.656)) = e^(−0.601 × 0.9764) = e^(−0.5868) = 0.5564
Cystatin C component:
- Scys/0.8 = 1.13 / 0.8 = 1.4125 → this is > 1, so:
- min(1.4125, 1)^(−0.375) = 1^(−0.375) = 1.000
- max(1.4125, 1)^(−0.711) = 1.4125^(−0.711) = e^(−0.711 × ln(1.4125)) = e^(−0.711 × 0.3455) = e^(−0.2456) = 0.7823
Age factor:
- 0.9938^40 = e^(40 × ln(0.9938)) = e^(40 × −0.006219) = e^(−0.2488) = 0.7797
Final eGFR:
- 135 × 1.000 × 0.5564 × 1.000 × 0.7823 × 0.7797
- = 135 × 0.5564 × 0.7823 × 0.7797
- = 135 × 0.5564 × 0.6101
- = 135 × 0.3395
- = 45.8 mL/min/1.73 m²
Result
| Formula | eGFR |
|---|---|
| CKD-EPI Creatinine–Cystatin C 2021 | ~46 mL/min/1.73 m² |
CKD Staging (KDIGO 2022)
| eGFR (mL/min/1.73 m²) | Stage |
|---|---|
| ≥ 90 | G1 – Normal |
| 60–89 | G2 – Mildly decreased |
| 45–59 | G3a – Mildly to moderately decreased |
| 30–44 | G3b – Moderately to severely decreased |
| 15–29 | G4 – Severely decreased |
| < 15 | G5 – Kidney failure |
This patient falls in CKD Stage G3a (eGFR 45–59), indicating mildly to moderately decreased kidney function.
Clinical Notes
- The CKD-EPI 2021 combined creatinine + cystatin C equation has the lowest bias and highest precision of all GFR estimating equations, particularly in patients with eGFR 45–75 range where creatinine alone can be misleading (affected by muscle mass).
- Cystatin C is less influenced by muscle mass, diet, or race — making the combined equation preferred for confirmatory testing.
- KDIGO recommends confirming CKD with ≥2 measurements over ≥3 months plus assessment of albuminuria.
- At eGFR ~46, referral to nephrology and monitoring for CKD complications (anemia, mineral-bone disorder, hypertension management) is appropriate.
But urine microalbumin is normal
Good observation. Here's how normal albuminuria changes the interpretation:
KDIGO CKD Classification — Updated with Albuminuria
KDIGO 2022 classifies CKD using both eGFR (G stage) and albuminuria (A stage).
Albuminuria Categories
| Category | ACR (mg/g) or AER (mg/24h) | Description |
|---|---|---|
| A1 | < 30 | Normal to mildly increased |
| A2 | 30–300 | Moderately increased |
| A3 | > 300 | Severely increased |
This patient → A1 (normal microalbumin)
This Patient's Full CKD Profile
| Parameter | Value | Stage |
|---|---|---|
| eGFR | ~46 mL/min/1.73 m² | G3a |
| Urine microalbumin | Normal (< 30 mg/g) | A1 |
| Combined CKD stage | G3a A1 |
Does Normal Albuminuria Change the Diagnosis?
Yes — it has important implications:
-
CKD diagnosis still stands — CKD requires either reduced eGFR (<60) OR markers of kidney damage for >3 months. An eGFR of ~46 for ≥3 months is sufficient to diagnose CKD G3a even without albuminuria.
-
Prognosis is better — The KDIGO heat map places G3a/A1 in the "moderately increased risk" (yellow) category — much better than G3a/A2 or G3a/A3 which carry higher risk.
-
Consider non-proteinuric causes of CKD:
- Hypertensive nephrosclerosis
- Renovascular disease / renal artery stenosis
- Tubulointerstitial nephritis
- Obstructive uropathy
- Adult polycystic kidney disease (ADPKD)
- Ischemic nephropathy
- These tend to cause reduced GFR without heavy proteinuria
-
Also reconsider whether eGFR is truly reduced — at G3a/A1, it is worth ruling out:
- Reduced muscle mass (low creatinine generation) — though cystatin C here also points to impaired function
- Pre-renal factors (dehydration, NSAID use, ACEi/ARB)
- Repeat eGFR in 3 months to confirm chronicity
Recommended Next Steps
| Investigation | Reason |
|---|---|
| Repeat eGFR in 3 months | Confirm chronicity (required for CKD diagnosis) |
| Urine PCR / full urinalysis + microscopy | Look for casts, hematuria (glomerular vs tubular) |
| Renal ultrasound | Size, echogenicity, obstruction, cysts |
| BP measurement | Hypertensive nephrosclerosis most common non-proteinuric cause |
| Electrolytes, HCO₃, phosphate, PTH, Hb | CKD complication screen |
| Fasting glucose / HbA1c | Exclude early diabetic nephropathy (can be non-proteinuric early on) |
Bottom line: eGFR ~46 + normal albuminuria = CKD G3a A1, moderate risk category. The absence of proteinuria shifts the differential away from glomerular disease toward vascular, obstructive, or tubulointerstitial causes. Chronicity still needs confirmation with a repeat measurement at 3 months.
Microalbumin results 0.3 mg/L
Microalbumin 0.3 mg/L — Interpretation
The unit here is mg/L (concentration in a spot urine sample), not mg/g (ACR) or mg/24h.
Reference Range for Urine Microalbumin (concentration)
| Category | Concentration (mg/L) |
|---|---|
| Normal | < 20 mg/L |
| Microalbuminuria | 20 – 200 mg/L |
| Macroalbuminuria | > 200 mg/L |
0.3 mg/L is well within normal range — confirmed A1.
Important Caveat: Concentration vs. ACR
Urine albumin concentration alone can be affected by hydration status:
- Dilute urine → falsely low concentration
- Concentrated urine → falsely high concentration
For this reason, Albumin-to-Creatinine Ratio (ACR) on a spot urine sample is the preferred method per KDIGO guidelines, as it corrects for urine concentration.
| Method | Preferred? |
|---|---|
| Urine microalbumin concentration (mg/L) | Less preferred — affected by dilution |
| ACR (mg/g or mg/mmol) | Preferred — KDIGO standard |
| 24-hour urine albumin (mg/24h) | Acceptable but cumbersome |
If a urine creatinine was measured alongside this sample, calculating the ACR would give a more reliable result.
Updated Patient Summary
| Parameter | Value | Interpretation |
|---|---|---|
| Age/Sex | 40M | — |
| Serum Creatinine | 2.39 mg/dL | Elevated |
| Serum Cystatin C | 1.13 mg/L | Elevated |
| eGFR (CKD-EPI 2021) | ~46 mL/min/1.73 m² | G3a |
| Urine Microalbumin | 0.3 mg/L | Normal → A1 |
| CKD Stage | G3a A1 |
Key Clinical Point
A urine microalbumin of 0.3 mg/L is extremely low — this is not a borderline result. Even accounting for any dilution effect, this strongly supports A1 (normoalbuminuria).
This makes non-glomerular causes of CKD much more likely in this patient:
- Hypertensive nephrosclerosis (most common in a 40-year-old male)
- Renovascular/ischemic nephropathy
- Chronic tubulointerstitial nephritis
- ADPKD (renal ultrasound is essential)
Recommend obtaining an ACR on a morning void sample to formally confirm, alongside a renal ultrasound as the next most informative investigation.
What is concept of cystatin c in kidney disease
Finding Sources
Searching the Web
cystatin C biomarker kidney disease GFR estimation concept
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Reading File
Cystatin C in Kidney Disease — Complete Concept
1. What Is Cystatin C?
Cystatin C is a low-molecular-weight (13 kDa) cysteine protease inhibitor produced at a constant rate by all nucleated cells in the body. It belongs to the cystatin superfamily of proteins.
Key biochemical features:
- Non-glycosylated basic protein
- Encoded by the CST3 gene
- Produced continuously regardless of metabolic state
- Found in serum, urine, CSF, and other body fluids
2. Why Is It an Ideal GFR Marker? — The "Ideal Filtration Marker" Criteria
| Property | Cystatin C | Creatinine |
|---|---|---|
| Constant production rate | ✅ Yes | ✗ Variable (muscle-dependent) |
| Freely filtered by glomerulus | ✅ Yes (small size) | ✅ Yes |
| Not secreted by tubules | ✅ Yes | ✗ Some secretion (~10–40%) |
| Tubular reabsorption | ✅ Reabsorbed & catabolized (none in urine normally) | ✗ None |
| Independent of muscle mass | ✅ Largely yes | ✗ Directly dependent |
| Independent of diet | ✅ Yes | ✗ Meat intake affects it |
| Independent of sex | ✅ Yes | ✗ Males higher |
| Independent of race | ✅ Yes | ✗ Race correction previously needed |
Because it is freely filtered, not secreted, and completely catabolized in proximal tubule cells, serum cystatin C concentration directly reflects GFR — when GFR falls, cystatin C accumulates in the blood.
"Proximal tubule cells reabsorb and catabolize the filtered cystatin C so that little is normally excreted in the urine... cystatin C measurement cannot be used as a conventional urinary excretory marker for GFR." — Brenner & Rector's The Kidney
3. Normal Reference Range
| Population | Cystatin C (mg/L) |
|---|---|
| Young adults (23–50 years) | 0.6 – 1.1 mg/L |
| Neonates (< 3 months) | Higher (renal immaturity) |
| After 1 year of age | Adult levels |
Within-person biological variation (CV) is only ~4–6.8%, making it a very stable, reproducible marker for monitoring GFR changes over time.
4. Cystatin C vs. Creatinine — The "Creatinine-Blind Spot"
Creatinine has a well-known limitation called the "creatinine-blind zone" — GFR can fall significantly before serum creatinine rises above normal, because:
- Muscle mass varies widely (athletes vs. elderly, amputees, malnutrition)
- Tubular secretion increases as GFR falls, masking the true drop
- Diet (meat intake) elevates creatinine independently
Cystatin C detects early GFR decline more sensitively, particularly in:
| Patient Group | Why Creatinine Fails | Cystatin C Advantage |
|---|---|---|
| Elderly / sarcopenic | Low muscle mass → low creatinine despite poor GFR | Unaffected by muscle |
| Cachexia / malnutrition | Same as above | More reliable |
| Amputees / paraplegia | Severely reduced muscle mass | Accurate |
| Athletes / bodybuilders | High muscle mass → high creatinine, overestimates damage | Unaffected |
| Creatine supplement users | Raises creatinine directly | Unaffected |
| Obese patients | Creatinine underestimates impairment | More accurate |
| Rapid AKI | Creatinine lags by hours | Cystatin C rises sooner (shorter half-life) |
5. GFR Estimating Equations Using Cystatin C
Three main equations:
| Equation | Variables Used | Notes |
|---|---|---|
| CKD-EPI Cystatin C 2012 | Cystatin C + age + sex | Better than creatinine alone |
| CKD-EPI Creatinine–Cystatin C 2021 | Creatinine + Cystatin C + age + sex | Most accurate — recommended |
| Caucasian Asian Pediatric Adult (CAPA) | Cystatin C | Used in children |
The combined 2021 equation (used in your patient's calculation) is the current gold standard — it has greater precision and accuracy than either marker alone, correctly classifying GFR <60 in significantly more patients.
6. KDIGO Clinical Indications for Cystatin C
Per KDIGO 2022 guidelines, measure cystatin C when:
-
eGFR (creatinine) is 45–59 mL/min/1.73 m² WITHOUT markers of kidney damage (albuminuria, hematuria, etc.) — to confirm or refute CKD diagnosis
- If cystatin C–based eGFR is also <60 → CKD confirmed
- If cystatin C–based eGFR is ≥60 → CKD NOT confirmed (likely falsely low creatinine-eGFR)
-
Suspected discordance between creatinine-eGFR and true GFR (muscle extremes)
-
Drug dosing decisions — particularly for renally cleared drugs in patients where creatinine is unreliable
-
Cardiovascular risk prediction — cystatin C predicts death and kidney failure independent of GFR accuracy (carries prognostic information beyond just GFR)
7. Non-GFR Determinants — Limitations of Cystatin C
Cystatin C is not perfectly GFR-specific. It can be elevated by:
| Factor | Effect on Cystatin C |
|---|---|
| Corticosteroids | ↑ (major confounder — limits use post-transplant) |
| Hyperthyroidism | ↑ |
| Hypothyroidism | ↓ (may mask reduced GFR) |
| Obesity | ↑ |
| Diabetes | ↑ |
| Smoking | ↑ |
| High CRP / inflammation | ↑ |
| Malignancy | ↑ |
8. Cystatin C in Acute Kidney Injury (AKI)
Because cystatin C has a shorter half-life than creatinine:
- Serum cystatin C rises earlier when GFR falls acutely
- Returns to normal sooner during renal recovery
- Urinary cystatin C appears when tubular damage occurs (normally it is fully catabolized — its presence in urine signals proximal tubule dysfunction)
9. Cardiovascular Risk — Beyond GFR
A unique and important property: cystatin C predicts cardiovascular events, end-stage renal disease, and all-cause mortality independent of its GFR-estimating accuracy. This may relate to its role as a cysteine protease inhibitor in vascular biology.
Summary Table
| Feature | Detail |
|---|---|
| Size | 13 kDa |
| Source | All nucleated cells — constant rate |
| Handling | Freely filtered → tubular catabolism → none in urine normally |
| Normal range | 0.6–1.1 mg/L |
| Best equation | CKD-EPI Creatinine–Cystatin C 2021 |
| Key advantage over creatinine | Independent of muscle mass, diet, sex, race |
| Key KDIGO indication | Confirm CKD when creatinine-eGFR 45–59 + no albuminuria |
| Key limitation | Elevated by steroids, thyroid disease, inflammation |
| AKI role | Earlier rise and fall vs. creatinine; urinary cystatin C = tubular damage |
Sources: Brenner & Rector's The Kidney; Tietz Textbook of Laboratory Medicine, 7th Ed.
So I increased in CKD or decrease in CKD
Cystatin C in CKD — Increased or Decreased?
Serum Cystatin C is INCREASED in CKD.
The Simple Logic
GFR falls → Less filtration → Cystatin C accumulates in blood → Serum Cystatin C ↑
Cystatin C is produced at a constant rate by all nucleated cells. It can only leave the body by being filtered at the glomerulus. When the kidney filters less (low GFR), cystatin C builds up in the serum — just like creatinine and urea.
Inverse Relationship: Cystatin C vs. GFR
| GFR Status | Serum Cystatin C |
|---|---|
| Normal GFR (>90) | Normal (0.6–1.1 mg/L) |
| Mildly reduced GFR (60–89) | Slightly ↑ |
| Moderately reduced GFR (30–59) | Moderately ↑↑ |
| Severely reduced GFR (15–29) | Markedly ↑↑↑ |
| Kidney failure (<15) | Very high ↑↑↑↑ |
Serum cystatin C ↑ = GFR ↓ (worse kidney function)
Your Patient's Values in Context
| Marker | Patient Value | Normal | Interpretation |
|---|---|---|---|
| Serum Cystatin C | 1.13 mg/L | 0.6–1.1 mg/L | Slightly ↑ above upper limit |
| Serum Creatinine | 2.39 mg/dL | 0.7–1.2 mg/dL | Markedly ↑ |
| eGFR | ~46 mL/min/1.73 m² | >60 | ↓ — reduced |
Both markers are elevated → consistent with reduced GFR → confirms CKD G3a.
Memory Aid
"As GFR goes DOWN, Cystatin C goes UP" Think of it like a bucket with a blocked drain — if water (cystatin C) is poured in at a constant rate but the drain (glomerulus) is blocked, the bucket fills up.
But why his EGFR is coming low. ?
Great clinical question. Let's think through this systematically.
Why Is His eGFR Low (~46)?
This 40-year-old male has:
- Creatinine 2.39 mg/dL (markedly elevated)
- Cystatin C 1.13 mg/L (mildly elevated)
- Microalbumin 0.3 mg/L (normal — A1)
- eGFR ~46 mL/min/1.73 m²
The low eGFR means his kidneys are filtering at roughly half the expected rate for his age.
The Key Diagnostic Clue: No Proteinuria
The absence of albuminuria is the most important finding here. It narrows the differential significantly:
| Pattern | Likely Cause |
|---|---|
| Low eGFR + heavy proteinuria | Glomerular disease (diabetic nephropathy, glomerulonephritis) |
| Low eGFR + no/minimal proteinuria | Vascular, tubulo-interstitial, obstructive, or hereditary cause |
This patient fits the second pattern.
Differential Diagnosis — Why eGFR is Low Without Proteinuria
1. Hypertensive Nephrosclerosis (Most Common)
- Hypertension damages small renal vessels (afferent arterioles) → reduced glomerular perfusion → low GFR
- Arteriolar thickening → ischemic glomeruli → nephron loss
- Classic in a 40-year-old male, especially if hypertension is long-standing or poorly controlled
- Proteinuria is typically absent or minimal
2. Renovascular Disease / Renal Artery Stenosis
- Atherosclerosis or fibromuscular dysplasia narrows renal artery → reduced perfusion pressure → low GFR
- Activates RAAS → hypertension + reduced GFR
- Hallmark: refractory hypertension + rising creatinine on ACEi/ARB
3. Chronic Tubulointerstitial Nephritis (CTIN)
- Inflammation/fibrosis of renal interstitium and tubules
- Causes: NSAIDs (long-term), analgesic abuse, lithium, aristolochic acid, hypercalcemia, uric acid, recurrent infections
- Tubular dysfunction prominent — GFR falls without glomerular involvement → no proteinuria
- Ask about NSAID use, pain killer abuse, herbal medicines
4. Obstructive Uropathy (Chronic)
- Chronic obstruction (urinary stones, BPH, posterior urethral valves, retroperitoneal fibrosis) → back-pressure → tubular atrophy
- Renal ultrasound essential — will show hydronephrosis
- Relatively painless if chronic
5. Adult Polycystic Kidney Disease (ADPKD)
- Autosomal dominant, usually presents in 3rd–5th decade
- Cysts gradually replace nephrons → GFR falls
- Minimal proteinuria (non-glomerular mechanism)
- Renal ultrasound shows large kidneys with multiple cysts
- Often family history of kidney disease or early dialysis
6. Ischemic Nephropathy
- Systemic cardiovascular disease → generalized reduced renal perfusion
- Common in patients with coronary artery disease, heart failure, peripheral vascular disease
7. Pre-renal (Functional — Reversible)
- Dehydration, low cardiac output, NSAIDs, ACEi/ARB use → reduced glomerular perfusion → low eGFR
- Important to exclude before diagnosing structural CKD
- If eGFR improves significantly after hydration/medication adjustment → not true CKD
Creatinine–Cystatin C Discordance — An Important Observation
Notice that in this patient:
- Creatinine is markedly elevated (2.39 vs. normal <1.2)
- Cystatin C is only mildly elevated (1.13 vs. normal <1.1)
This relative discordance suggests creatinine may be disproportionately elevated — possible reasons:
- High muscle mass (athletic 40-year-old male)
- High protein diet / creatine supplement use
- Reduced tubular secretion of creatinine with early CKD
The cystatin C–based component of the equation partially corrects for this, which is exactly why the combined equation was used.
Recommended Workup to Find the Cause
| Investigation | What It Finds |
|---|---|
| Renal ultrasound | Size, echogenicity, hydronephrosis, cysts (ADPKD) |
| Blood pressure measurement | Hypertensive nephrosclerosis |
| Urine microscopy + RBC casts | Glomerulonephritis (even without proteinuria) |
| Serum uric acid | Urate nephropathy |
| Fasting glucose / HbA1c | Early diabetic nephropathy |
| Serum electrolytes + bicarbonate | Tubular dysfunction (RTA) |
| Renal Doppler ultrasound | Renal artery stenosis |
| Drug/medication history | NSAIDs, lithium, herbal medicines |
| Family history | ADPKD, Alport syndrome |
| Repeat eGFR in 3 months | Confirm chronicity for CKD diagnosis |
Bottom Line
His eGFR is low because of reduced nephron filtration capacity, most likely from a non-glomerular cause given the absent proteinuria. In a 40-year-old male, hypertensive nephrosclerosis is the single most common explanation, followed by chronic tubulointerstitial disease and obstructive causes. A renal ultrasound and blood pressure assessment are the two most immediately informative next steps.
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