CPT Code 83735: Magnesium serum billing and coding guide

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Key takeaways

CPT Code 83735 describes measurement of magnesium in serum or plasma, classified under the 80000-series chemistry procedures.

Medicare covers 83735 under LCD L36700 only when the claim carries an approved ICD-10-CM diagnosis, so an unlisted code denies automatically.

The Clinical Laboratory Fee Schedule pays one flat national rate for 83735, currently around $5 to $7, with no locality or facility adjustment.

No standard chemistry panel includes magnesium, so 83735 is reported separately alongside 80048, 80053 and 80069.

Pabau’s Claim.MD integration submits the CMS-1500 claim, checks eligibility in real time, and tracks claim status and remittance advice.

CPT Code 83735 is the chemistry code for a quantitative magnesium measurement in serum or plasma. Medicare pays it under the Clinical Laboratory Fee Schedule at a single national rate of roughly $5 to $7. Coverage runs through LCD L36700, which pays the test only when the claim carries an ICD-10-CM diagnosis from the CMS-approved list.

Denials on this code come from a short list. Either the paired diagnosis is not on the approved list, or modifier 91 is missing on a same-day repeat. The third cause is a chart that never says why the test was ordered.

One assumption causes more trouble than any of those. Magnesium is not a component of the basic, comprehensive, or renal function panel. That makes 83735 its own claim line every time.

Code descriptor, category, and specimen type

CPT Code 83735 is a chemistry procedure in the American Medical Association’s 80000-series code range. The AMA CPT code set assigns it the short descriptor Magnesium; serum, indicating a quantitative measurement of magnesium concentration in a serum or plasma specimen.

The code covers the full quantitative assay, not a qualitative screen. A whole-blood magnesium measurement uses a different code. 83735 applies only when the specimen is serum or plasma collected by venipuncture or an equivalent method.

Clinical indications: when to order a serum magnesium test

Medicare’s Local Coverage Determination L36700 defines the clinical scenarios in which a serum magnesium test is considered medically necessary. Ordering outside these scenarios means the claim carries a diagnosis code that is not on the CMS-approved list, which triggers a denial before adjudication begins.

• Hypomagnesemia or suspected magnesium deficiency – patients with symptoms of low magnesium (muscle cramps, arrhythmias, seizures) or a history of conditions known to cause depletion
• Hypermagnesemia monitoring – patients receiving magnesium supplementation or at risk of elevated levels, including those with renal impairment
• Chronic kidney disease (CKD) – renal disease impairs magnesium excretion and requires periodic monitoring
• Malnutrition or malabsorption syndromes – including Crohn’s disease, celiac disease, short-bowel syndrome, and chronic alcohol use disorder
• Diuretic therapy monitoring – loop diuretics (furosemide, bumetanide) and thiazide diuretics cause urinary magnesium wasting
• Proton pump inhibitor (PPI) monitoring – the FDA has documented hypomagnesemia as a recognized adverse effect of long-term PPI use. Testing is indicated after three or more months of therapy
• Parenteral nutrition – patients receiving total parenteral nutrition require electrolyte monitoring that routinely includes magnesium
• Cardiac arrhythmia workup – hypomagnesemia can precipitate or exacerbate ventricular arrhythmias

Each of these scenarios maps to one or more ICD-10-CM codes that CMS has approved to support medical necessity for CPT 83735. The diagnosis code must reflect the clinical finding or monitoring rationale, not a generic or unrelated condition.

ICD-10-CM codes that support medical necessity

CMS Billing and Coding Article A57198 publishes the approved ICD-10-CM list for CPT 83735. The table below covers the primary supporting codes. Always verify against the current version of A57198 in the CMS Medicare Coverage Database, since the list is updated with each ICD-10 fiscal year release. Pairing 83735 with a code outside this list produces an automatic medical necessity denial.

ICD-10-CM codes that do not support medical necessity

CMS Article A57198 also lists codes that produce a denial when billed with CPT 83735. Routine screening without a documented clinical indication falls outside coverage, so codes for general health screenings, wellness exams, or administrative purposes are not accepted.

Using Z00.00 or Z00.01, the general adult examination codes, as the sole paired diagnosis produces an automatic non-covered denial. The test has to be ordered in response to a specific clinical finding or monitoring protocol, with a diagnosis code that reflects it.

Medicare reimbursement rate for CPT 83735

Laboratory codes, including CPT 83735, are paid under the Clinical Laboratory Fee Schedule (CLFS), not the Medicare Physician Fee Schedule (MPFS). The distinction matters because the two schedules are built and published separately. The CMS Physician Fee Schedule lookup tool covers MPFS codes only, so it returns nothing useful for 83735.

2026 fee schedule and payer rates

The CLFS pays one national amount for each test. Since the PAMA reform took effect in 2018, lab codes carry no locality adjustment. A magnesium level pays the same in Los Angeles as it does in rural Iowa. Confirm the current figure in the CMS Clinical Laboratory Fee Schedule file rather than an RVU tool, which will not carry the code.

The electronic remittance advice from the payer confirms the allowed amount after adjudication. Reviewing it line by line is how an underpayment on a low-dollar lab code gets noticed at all.

Medicare coverage under LCD L36700

Local Coverage Determination L36700 governs Medicare coverage for serum magnesium testing. Billing and Coding Article A57198 carries the code lists and documentation guidance that accompany it. Both matter: the LCD defines the coverage policy, and the article defines how to bill against it.

Documentation requirements for medical necessity

The medical record must contain enough information to support the clinical rationale for ordering CPT 83735. CMS auditors assess documentation at the time of a claim review, not at the time of ordering. Thin documentation is the most common reason a covered test gets denied on appeal.

• Ordering rationale – a note entry stating why the test was ordered (e.g., “patient on furosemide 40mg daily; monitoring for diuretic-induced hypomagnesemia”)
• Supporting symptoms or diagnosis – documented symptoms consistent with magnesium dysregulation, or an established diagnosis that requires monitoring
• Prior results where relevant – if ordering a repeat test, document the prior result and the clinical decision it informed
• Medication list – for PPI or diuretic monitoring claims, the medication and duration must appear in the chart
• Plan of care connection – the test should connect to a documented treatment plan or monitoring protocol, not appear as a standalone order without clinical context

Bill type codes and revenue codes required for institutional billing are enumerated in CMS Article A57198. For Part B claims submitted by independent laboratories, refer to the Medicare contractor’s billing guidance for the applicable form and revenue code requirements.

Modifiers that apply to 83735

Modifiers clarify the circumstances under which CPT 83735 was performed. Using the wrong modifier, or omitting a required one, causes a rejection or a payment reduction. The AAPC Codify CPT lookup includes modifier guidance for individual codes alongside fee schedule data.

Modifier 91 is the most frequently omitted modifier on 83735 claims. When a clinician orders a repeat magnesium level to assess the response to IV replacement given earlier the same day, modifier 91 is required. Without it, the second claim line is rejected as a duplicate.

Related CPT codes often billed with 83735

Magnesium is rarely measured in isolation. Most ordering patterns involve a panel or an electrolyte workup, so CPT 83735 frequently appears alongside these chemistry codes. The CMS CPT/HCPCS code list and the NCCI edits govern which codes can be billed together.

The renal function panel is the one that trips people up, because coders often assume it carries magnesium. It does not. Its ten components are albumin, calcium, carbon dioxide, chloride, creatinine, glucose, phosphorus, potassium, sodium, and urea nitrogen.

The basic metabolic panel carries eight analytes and the comprehensive metabolic panel fourteen, and magnesium is in neither. No NCCI edit bundles 83735 into any of the three, so it is reported as its own line whenever it is ordered.

How to bill CPT 83735: step-by-step guide

The steps below consolidate the CMS coverage requirements, the NCCI considerations, and the documentation standards into one end-to-end process. Work through them in order at the point of ordering, rather than at the point of appeal.

1. Confirm clinical indication – verify the patient has a documented condition or monitoring need that maps to an approved ICD-10-CM code per LCD L36700. If no clinical indication exists, the test is not billable to Medicare.
2. Select the paired ICD-10-CM code – choose the most specific applicable code from the CMS A57198 approved list. Use E83.42 for documented hypomagnesemia, E83.41 for hypermagnesemia, N18.x for CKD monitoring, and Z79.899 for long-term PPI or diuretic therapy monitoring.
3. Record the place of service – the CLFS rate does not change with the setting. The POS code still has to be correct on the claim. Independent laboratory billing has its own requirements under Part B.
4. Apply applicable modifiers – add modifier 91 if 83735 is ordered more than once the same day with clinical justification. Add QW if the test is performed on a CLIA-waived device. Verify no NCCI edit pairs require modifier 59 for co-billed chemistry codes.
5. Confirm chart documentation – the ordering note must state the rationale, the relevant diagnosis, and, for repeat tests, the prior result. Documentation must be in the chart before the claim is submitted.
6. Submit the claim – include CPT 83735, the paired ICD-10-CM code, the correct modifiers, and the place of service. Pabau’s integration with the Claim.MD clearinghouse sends the CMS-1500 claim electronically and returns the acceptance or rejection to the patient record.
7. Monitor remittance – review the ERA for the allowed amount, any reduction codes, and denial reason codes. Verifying eligibility before the visit removes the coverage denials caused by inactive policies or a wrong subscriber ID.

Common billing errors and denial reasons

Most 83735 denials fall into a short list of avoidable categories. Knowing which denial codes come back most often on lab claims tells you which check to add first. The table below maps each error to the denial it produces and how to prevent it.

A clean claim for CPT 83735 carries the correct code, an approved paired ICD-10 code, any required modifier, and chart documentation that supports the diagnosis. Logging denial reason codes by CPT code shows which of those four fails most often in your own practice.

How Pabau handles lab claim submission and tracking

In most practices the magnesium order lives in one system and the claim for it lives in another. Someone re-keys the code, the diagnosis, and the modifier into a billing portal. The outcome then gets chased by phone or on a payer website days later.

Pabau, an all-in-one practice management system, connects to the Claim.MD clearinghouse instead. The CMS-1500 claim goes out electronically from the patient record. Coverage is checked in real time before the visit, and the claim’s status and remittance advice come back to the same record.

That does not pick the code for you. The ICD-10 pairing, the modifier decision, and the NCCI check stay with your coder, where LCD L36700 puts them. Pabau’s claims management software removes the re-keying and the blind wait between submission and payment. That wait is where low-dollar lab claims usually get lost.

Stop losing lab claims between systems

Pabau connects to Claim.MD to submit CMS-1500 lab claims, check eligibility in real time, and track status and remittance advice against the patient record. See how it runs in your practice.

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Conclusion

CPT 83735 is a straightforward chemistry code with predictable denial patterns. Three things decide whether it pays. They are the paired ICD-10 code, modifier 91 on same-day repeats, and a chart note stating why the test was ordered.

Fix those three at the point of ordering and the code largely bills itself. The payment is small enough that reworking a denial costs more than the claim is worth. That arithmetic is the whole argument for getting it right the first time.

Book a demo to see how Pabau submits and tracks lab claims like 83735 without pulling your team into a second billing portal.

Continue your research

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Looking to reduce claim rework across all your lab codes? How Claim.MD clearinghouse works covers real-time eligibility checks and ERA processing that apply to every chemistry code you submit.

Want to benchmark your denial rate against industry norms? Best medical billing software in the US compares tools that include denial analytics and reporting for lab-heavy practices.

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