McMahon score: how the rhabdomyolysis score adds up

The McMahon score adds points for eight features in rhabdomyolysis: age, sex, initial creatinine, calcium, CPK, phosphate and bicarbonate, and the cause of the muscle breakdown. Totals run from 0 to a theoretical 19, in half-point steps for some items.

Not a medical device. Not a diagnosis or dosing recommendation.

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McMahon GM et al. · 2013
The McMahon score formula
Worked examples
Frequently asked questions

What is the McMahon score?

Gearoid McMahon, Xiaoxi Zeng and Sushrut Waikar published the score in JAMA Internal Medicine in 2013. It was built in adults with rhabdomyolysis, the breakdown of skeletal muscle, to estimate the chance of needing kidney replacement or dying in hospital.

Magent's calculator applies the point table from the paper (Table 3) to the values you enter and returns the numeric total. It doesn't attach a named risk class to the total.

This page explains the arithmetic. It doesn't say what a total means for a particular person.

The McMahon score formula

Age             ≤50 → 0 · >50 to ≤70 → 1.5 · >70 to ≤80 → 2.5 · >80 → 3
Female sex      1
Creatinine      <1.4 → 0 · 1.4 to 2.2 → 1.5 · >2.2 → 3        (mg/dL)
Calcium         <7.5 → 2                                       (mg/dL)
CPK             >40000 → 2                                     (U/L)
Cause           not seizure, syncope, exercise, statin or myositis → 3
Phosphate       <4.0 → 0 · 4.0 to 5.4 → 1.5 · >5.4 → 3         (mg/dL)
Bicarbonate     <19 → 2                                        (mEq/L)
McMahon score = sum of the points · maximum 19

Exact expression the calculator uses:

McMahon = age(≤50→0, >50–≤70→1.5, >70–≤80→2.5, >80→3) + female(1) + Cr_mg_dL(<1.4→0, 1.4–2.2→1.5, >2.2→3) + Ca_mg_dL(<7.5→2) + CPK_U_L(>40000→2) + origin_not_seizure_syncope_exercise_statin_myositis(3) + PO4_mg_dL(<4.0→0, 4.0–5.4→1.5, >5.4→3) + HCO3_mEq_L(<19→2); McMahon 2013 Table 3 derivation; max 19

Inputs

Input	Meaning	Accepted range
`age`	Age in years, a whole number	18 to 120
`sex`	female or male	female, male
`creatinine_mg_dl`	Initial serum creatinine, mg/dL	0.1 to 20
`creatinine_umol_l`	Or initial creatinine in µmol/L, divided by 88.42	9 to 1768
`calcium_mg_dl`	Initial calcium, mg/dL	4 to 16
`cpk_u_l`	Initial creatine phosphokinase (CPK), U/L	1 to 500000
`low_risk_origin`	true if the cause is seizure, syncope, exercise, statin or myositis	true or false
`phosphate_mg_dl`	Initial phosphate, mg/dL	0.5 to 20
`bicarbonate_mmol_l`	Initial bicarbonate, mEq/L or mmol/L	2 to 45

How the scoring works

Age: 50 or under scores 0. Over 50 up to 70 scores 1.5. Over 70 up to 80 scores 2.5. Over 80 scores 3.
Sex: female scores 1, male 0.
Creatinine: under 1.4 scores 0. 1.4 to 2.2, both ends included, scores 1.5. Over 2.2 scores 3.
Calcium: under 7.5 scores 2. A calcium of exactly 7.5 scores 0.
CPK: over 40000 scores 2. Exactly 40000 scores 0.
Cause: seizure, syncope, exercise, statin or myositis scores 0 (low_risk_origin true). Any other cause scores 3.
Phosphate: under 4.0 scores 0. 4.0 to 5.4, both ends included, scores 1.5. Over 5.4 scores 3.
Bicarbonate: under 19 scores 2. Exactly 19 scores 0.
Add the eight items.

The theoretical maximum, adding every top value, is 19. The paper observed totals from 0 to 17.5.

Units

Creatinine, calcium and phosphate are in mg/dL, CPK in U/L, and bicarbonate in mEq/L (the same number as mmol/L). Creatinine can also be given in µmol/L; the calculator divides by 88.42 to get mg/dL. Age must be a whole number. A missing value or one outside its range is rejected with an error. The calculator returns each input used, the points for each item, the total and the maximum of 19.

Worked examples

These use Magent's own expected values for the calculator. Each starts from a baseline that scores 0 (age 40, male, creatinine 1.0, calcium 9.0, CPK 2000, a low-risk cause, phosphate 3.5, bicarbonate 24) and changes one thing.

Example 1: the zero baseline

Age 40 → 0, male → 0, creatinine 1.0 → 0, calcium 9.0 → 0, CPK 2000 → 0, low-risk cause → 0, phosphate 3.5 → 0, bicarbonate 24 → 0.
Total 0 out of 19.

Example 2: age at each step

Age 51 → 1.5. Age 70 → 1.5.
Age 71 → 2.5. Age 80 → 2.5.
Age 81 → 3.

Example 3: creatinine at each edge

Creatinine 1.4 → 1.5. Creatinine 2.2 → 1.5.
Creatinine 2.3 → 3.

Example 4: phosphate at each edge

Phosphate 4.0 → 1.5. Phosphate 5.4 → 1.5.
Phosphate 5.5 → 3.

Example 5: the single-step items

Female instead of male → 1.
Calcium 7.4 → 2.
CPK 40001 → 2.
A cause other than seizure, syncope, exercise, statin or myositis → 3.
Bicarbonate 18 → 2.

History of the McMahon score

The score comes from McMahon GM, Zeng X and Waikar SS, A risk prediction score for kidney failure or mortality in rhabdomyolysis, JAMA Internal Medicine, 2013 (volume 173, issue 19, pages 1821–1828). The abstract describes a retrospective cohort study at two Boston teaching hospitals. The outcome was a composite of kidney replacement with dialysis or death in hospital. The score was derived in patients from one cohort and then applied to patients from the other.

When it's used

The score was built from values at the start of a hospital stay in adults with rhabdomyolysis, which is why each lab input is labeled "initial".

Where it's used

The 2013 study took place in hospitalized adults. All the inputs are routine blood tests plus age, sex and the cause of the muscle injury.

Who uses it

The person using the calculator enters the lab values and decides whether the cause is one of the five low-risk causes. The calculator applies the cutoffs and adds the points.

Limits of the calculation

The calculator doesn't run lab tests or decide the cause. Every input comes from the person entering it.
It returns a number, not a named risk class.
The point table is the derivation table from the 2013 paper, built in adults with rhabdomyolysis.
It isn't an order to start dialysis.
This page doesn't say what to do with a total.

Clinical decisions belong to a qualified clinician who knows the patient.

Creatinine clearance (Cockcroft–Gault) estimates kidney function in mL/min from creatinine, age, weight and sex. The McMahon score uses creatinine as one scored item among eight.
FENa works out the fractional excretion of sodium from plasma and urine sodium and creatinine.

Glossary

Rhabdomyolysis: breakdown of skeletal muscle.
CPK: creatine phosphokinase, a muscle enzyme, in U/L.
Low-risk cause: seizure, syncope, exercise, statin or myositis; scores 0.
Initial value: the first measured value, as used in the 2013 paper.

Common mistakes

Scoring the low-risk causes. Seizure, syncope, exercise, statin and myositis score 0; any other cause scores 3.
Misreading the edges. Creatinine 2.2 and phosphate 5.4 still score 1.5. Calcium 7.5, CPK 40000 and bicarbonate 19 score 0.
Entering creatinine in µmol/L as mg/dL. Use the µmol/L input and the calculator divides by 88.42.
Rounding the total. Half points are part of the published table.

Source

McMahon GM, Zeng X, Waikar SS. A risk prediction score for kidney failure or mortality in rhabdomyolysis. JAMA Intern Med. 2013;173(19):1821-1828. DOI: 10.1001/jamainternmed.2013.9774. PMID: 24000014

Frequently asked questions

How is the McMahon score calculated?

Add points for age, female sex, initial creatinine, calcium, CPK, phosphate and bicarbonate, and 3 points if the cause isn't seizure, syncope, exercise, statin or myositis.

What is the highest McMahon score?

19 in theory, adding every top value. The 2013 paper observed totals from 0 to 17.5.

Why does the McMahon score use half points?

The point values for age, creatinine and phosphate in the paper's Table 3 include 1.5 and 2.5, and the calculator keeps them.

Can I enter creatinine in µmol/L?

Yes. The calculator divides by 88.42 to get mg/dL before scoring.

Does the calculator return a risk group?

No. It returns the total.