CPT (Current Procedural Terminology) codes are five-digit numbers that describe every medical service or procedure. Your doctor, hospital, and insurance company use them to communicate exactly what was done. Every line on your medical bill has a CPT code, and knowing the code lets you look up what Medicare pays for that same service.

Why do providers charge so much more than Medicare pays?

Providers set their own "chargemaster" prices, which are essentially list prices. Medicare pays a fixed rate based on the resources each service actually requires (the RVU system for procedures, or the Clinical Lab Fee Schedule for lab tests). Commercial insurers typically pay 150% to 250% of Medicare rates. Uninsured patients are often billed the full chargemaster price, which can be 2x to 30x what Medicare pays.

Can I use Medicare rates to negotiate my bill?

Yes. Medicare rates are the closest thing to a "fair price" benchmark in U.S. healthcare. When negotiating, ask the provider what they accept from Medicare for the same CPT code. Most providers will accept 150% to 200% of Medicare for cash-pay patients, which is still far less than their chargemaster price. Many hospital financial assistance policies also use a multiple of Medicare as their discount floor.

How are Medicare rates calculated?

For most procedures, Medicare uses the Resource-Based Relative Value Scale (RBRVS). Each CPT code gets three RVU components: Work (physician time and skill), Practice Expense (overhead), and Malpractice. These are multiplied by geographic adjustment factors (GPCIs) and a conversion factor ($33.4009 in 2026). Lab tests (80000-89999 range) use a separate Clinical Laboratory Fee Schedule with flat national rates.

CPT 83735

Magnesium Level (Blood Test)

CPT 83735 is a serum magnesium test that measures this critical electrolyte in your blood. Medicare pays approximately $5 to $7 for this test, but providers charge an average of $37.73. The key fact most patients do not know: magnesium is NOT included in either the Basic Metabolic Panel (BMP) or the Comprehensive Metabolic Panel (CMP), so it always generates a separate charge on top of your routine blood work.

Updated May 2026Source: CMS Clinical Lab Fee Schedule

CPT 83735 at a Glance

Medicare CLFS rate: ~$5 to $7
Average provider charge: $37.73
Markup: ~6x over Medicare rate
Direct-to-consumer price: $10 to $25

Test type: Single analyte (electrolyte)
Beneficiaries (2023): 1.77 million
Fee schedule: Clinical Laboratory (CLFS)
Rate type: National (no geographic adjustment)

How Lab Pricing Works (Clinical Laboratory Fee Schedule)

Unlike physician services that use RVUs and geographic adjustments, lab tests are priced under the Medicare Clinical Laboratory Fee Schedule (CLFS). The CLFS sets a single national rate for each lab test. There are no RVU components and no geographic cost adjustments. A magnesium test costs Medicare the same amount at any lab in the country.

Metric	Value
Medicare CLFS Rate	~$5 to $7
Average Provider Charge	$37.73
Markup Ratio	~6x
Pricing Method	National rate (CLFS), no geographic variation

The hidden add-on charge: Because magnesium is not part of any standard metabolic panel, every time your doctor orders "a CMP plus magnesium," you are getting two separate charges: the CMP (~$60 at provider rates) and the magnesium ($38). This is legitimate billing, but many patients do not realize magnesium is a separate test with a separate fee until they see the itemized bill.

Lab tests are priced under the Clinical Laboratory Fee Schedule, not the Physician Fee Schedule. Medicare lab rates are set nationally and do not vary by geographic location.

What Does a Magnesium Test Measure?

Magnesium is an essential mineral involved in over 300 enzyme reactions in your body. It is critical for muscle function, nerve signaling, heart rhythm, bone strength, and blood sugar regulation. A serum magnesium test measures the amount of magnesium in your blood.

When Magnesium Is Ordered

Taking diuretics (water pills)
Taking proton pump inhibitors (PPIs like omeprazole)
Muscle cramps or spasms
Irregular heartbeat (arrhythmia)
Diabetes management
Chronic kidney disease
Potassium or calcium that stays low despite treatment

Important Limitation

Blood (serum) magnesium only reflects about 1% of your total body magnesium. Most magnesium is stored in bones and inside cells. This means:

A normal blood level does NOT guarantee adequate total body magnesium
You can be magnesium-deficient with a "normal" serum result
A low serum level is significant (true deficiency)
RBC magnesium is slightly better but still imperfect

Despite its limitations, serum magnesium remains the standard test because it is inexpensive, widely available, and clinically useful when levels are low. If your doctor suspects magnesium deficiency despite a normal serum level, they may trial magnesium supplementation based on symptoms.

Where to Get a Magnesium Test for Less

At $37.73 average charge, magnesium is not the most expensive lab test. But it adds up when combined with other tests, and the savings from using an independent lab are still proportionally large:

Direct-to-Consumer Labs: $10 to $25

Services like Ulta Lab Tests, Walk-In Lab, and Jason Health offer magnesium testing for $10 to $25. Some offer it as an add-on to metabolic panels for just a few dollars more. You pay online, visit a nearby lab for the blood draw, and receive results electronically.

Independent Labs (with doctor's order): $10 to $20

If your doctor orders magnesium along with other labs, make sure the order goes to Quest or LabCorp rather than a hospital lab. At an independent lab, the magnesium add-on is often just $10 to $20.

Hospital Outpatient Labs: $30 to $60+

Hospital labs charge the most, often $30 to $60 for a magnesium test alone. Combined with a CMP and other add-on tests, a hospital lab visit can quickly total $150 or more for what could cost $30 to $50 at an independent lab.

Tip for HDHP patients: If your doctor orders a CMP plus magnesium and you have not met your deductible, consider cash-pay at a direct-to-consumer lab. A CMP ($15) plus magnesium ($15) for $30 total may be cheaper than the negotiated rate ($60 CMP + $20 magnesium = $80) applied to your deductible. However, cash payments do not count toward your deductible.

What Insured Patients Actually Pay for a Magnesium Test

Insurance companies negotiate lab rates well below the $37.73 average charge. What you owe depends on your plan:

Your Situation	What You Likely Pay	How It Works
Copay plan (deductible met or N/A)	$0	Most plans cover lab work at 100% after deductible
Coinsurance plan (deductible met)	$1 to $4	20% of negotiated rate ($5 to $20)
High-deductible plan (deductible NOT met)	$5 to $20	Full negotiated rate applied to your deductible
Medicare Part B	$0	Medicare covers clinical lab tests at 100% (no coinsurance)

Common Billing Problems with CPT 83735

Surprise separate charge on top of metabolic panel

The most common "billing problem" with magnesium is not really an error, but a surprise. Patients assume magnesium is included in their metabolic panel because other electrolytes (sodium, potassium, chloride) are included. When they see a separate line item for magnesium, they think it is a billing mistake. It is not. Magnesium is always a separate charge (CPT 83735) because it is not part of BMP or CMP panels.

Magnesium ordered without clear clinical indication

Some providers add magnesium to every lab order as a routine add-on. While magnesium testing is appropriate for patients on certain medications (diuretics, PPIs) or with specific symptoms, it is not a universal screening test. If magnesium appears on every lab order and your levels are consistently normal, ask whether ongoing monitoring is necessary.

Hospital markup on a simple chemistry test

Magnesium is one of the simplest and cheapest tests to run on automated chemistry analyzers (pennies in reagent cost). Yet hospital labs may charge $40 to $60 for this single test. If your doctor's office is in a hospital system and labs auto-route to the hospital lab, the magnesium test alone could cost 3 to 4 times what an independent lab charges.

Frequently Asked Questions

How much does a magnesium blood test cost without insurance?

Without insurance, a magnesium test (CPT 83735) costs $20 to $60 at most hospitals and clinics, with the national average charge at $37.73. Direct-to-consumer labs offer magnesium testing for $10 to $25 without a doctor's order. Medicare pays approximately $5 to $7 for this test.

Is magnesium included in a Comprehensive Metabolic Panel (CMP)?

No. Magnesium is NOT included in either the Basic Metabolic Panel (BMP, CPT 80048) or the Comprehensive Metabolic Panel (CMP, CPT 80053). Despite being a commonly needed electrolyte, it must always be ordered separately with its own CPT code (83735) and its own charge. If your doctor orders "a CMP plus magnesium," you will see two line items on your bill.

Does a normal magnesium blood test mean my magnesium levels are fine?

Not necessarily. Blood (serum) magnesium represents only about 1% of total body magnesium, since most is stored in bones and cells. You can be functionally magnesium-deficient with a "normal" serum result. However, a low serum magnesium level is always significant and indicates true deficiency. Some doctors may recommend magnesium supplementation based on symptoms even if the blood test is normal.

Why would my doctor order a magnesium test?

Magnesium is commonly ordered for patients taking diuretics (water pills), proton pump inhibitors (PPIs like omeprazole or pantoprazole), or those experiencing muscle cramps, irregular heartbeat, diabetes, or chronic kidney disease. It is also checked when potassium or calcium levels remain low despite supplementation, since magnesium deficiency can prevent these other electrolytes from normalizing.

Need Help Lowering a Medical Bill?

Lab charges alone are often small, but they frequently appear on larger hospital or outpatient bills where the total adds up. CareRoute Bill Defense is a done-for-you bill reduction service that analyzes every code on your bill, identifies overcharges and unbundling errors, and negotiates on your behalf.

Learn about Bill Defense