How to Confirm Monitoring Labs and Follow-Up for Medications: A Practical Guide for Providers

When a patient is on a medication with a narrow therapeutic index-like digoxin, lithium, or vancomycin-getting the dose right isn’t just important. It’s life-or-death. Too little, and the drug doesn’t work. Too much, and it can cause toxicity, organ damage, or even death. That’s why medication monitoring isn’t optional. It’s a critical safety step. But here’s the problem: even if a lab runs the test, you can’t assume the result is accurate unless the lab has properly verified or validated the test. And if the test isn’t reliable, your follow-up decisions could be wrong. This guide cuts through the noise and shows you exactly how to confirm that monitoring labs are doing their job-and how to use those results safely.

What’s the Difference Between Verification and Validation?

Not all lab tests are created equal. If a lab uses a test kit made by Roche, Abbott, or Siemens that’s already been cleared by the FDA, they’re doing verification. That means they’re checking whether the test works the way the manufacturer says it does on their own equipment. They test precision, accuracy, and whether other substances in the blood interfere with the result.

If the lab makes its own test-like for a rare metabolite of clozapine or a new biologic drug-that’s validation. Validation is far more complex. It requires proving the test works from scratch: finding the lowest level it can detect, checking if it’s accurate across the full range of expected concentrations, and testing for interference from dozens of possible substances. Validation can take months. Verification takes weeks.

Why does this matter to you? Because labs that skip verification on FDA-cleared tests are cutting corners. A 2023 CAP survey found that 36% of labs didn’t properly verify the lower limit of quantitation for narrow therapeutic index drugs. That means they might miss when a patient’s level is just below the therapeutic range. For a patient on digoxin, that could mean they’re not getting enough medication-and their heart failure is worsening without anyone knowing.

The 9-Step Process Labs Must Follow

Every lab that does therapeutic drug monitoring should follow a clear, documented process. If they don’t, their results shouldn’t be trusted. Here’s what a proper verification or validation looks like:

1. Plan it out - The lab writes a formal plan listing what they’re testing, which guidelines they’re following (CLSI EP15-A3 or APHL toolkit), and what acceptance criteria they’ll use.
2. Get approval - The lab director must sign off before any testing begins. This isn’t a formality. It’s a legal requirement under CLIA.
3. Run the tests - For verification, they run at least 20 replicates over five days to check precision. They test with reference materials to check bias. They spike samples with common interfering drugs (like carbamazepine or phenytoin) to see if results get skewed.
4. Analyze the data - They use statistical tools to check if precision is within 1/4 of the total allowable error. For vancomycin, that means their results must be within ±15%. For digoxin, it’s ±10%.
5. Fix problems - If results don’t meet criteria, they troubleshoot. Maybe the instrument needs recalibration. Maybe the reagents are expired. Maybe the interference testing missed something.
6. Write the report - A full summary of all data, deviations, and conclusions. This document must be kept for at least two years.
7. Create SOPs - Standard Operating Procedures for how the test is performed daily. Everyone in the lab must be trained on them.
8. Final approval - The lab director reviews everything one last time and signs off.
9. Start testing patients - Only after all steps are done.

If a lab skips even one of these steps, their results are unreliable. You don’t need to be a lab scientist to ask: “Did you verify this test? Can I see the documentation?”

What to Ask the Lab Before Relying on Results

You don’t have to be an expert to spot red flags. Here’s what to ask when you’re unsure about a lab’s results:

• “Is this an FDA-cleared test or a lab-developed test?” - If it’s lab-developed, ask if they’ve validated it per CLSI or APHL standards.
• “What’s the total allowable error for this test?” - For lithium, it’s 10%. For tacrolimus, it’s 15%. If they don’t know, that’s a problem.
• “Did you test for interference from common drugs?” - For carbamazepine, cross-reactivity with its metabolites can cause false highs. For digoxin, antibodies from recent immunoglobulin therapy can cause false lows.
• “What’s your lower limit of quantitation?” - If it’s higher than the therapeutic range, you can’t tell if the patient is underdosed.
• “Do you report measurement uncertainty?” - Reputable labs now include this. It tells you how confident you can be in the number.

One provider in Ohio recently prescribed a higher dose of vancomycin because the lab reported a level of 18 mcg/mL. The patient developed acute kidney injury. When the provider asked for the verification report, the lab admitted they never tested linearity above 70 mcg/mL. The patient’s true level was 82 mcg/mL. The test couldn’t measure it. That’s not a lab error. That’s a failure of verification.

How to Spot a Lab That’s Cutting Corners

Some labs are under pressure to turn results around fast. Some are understaffed. Some just don’t care. Here are signs a lab isn’t doing verification right:

• They won’t share documentation - If they say “it’s proprietary” or “we don’t keep it,” walk away.
• Results are inconsistent - If the same patient’s level jumps from 1.2 to 2.8 ng/mL on digoxin with no change in dose, that’s not biology. That’s bad testing.
• The same test comes back from different labs with wildly different values - That’s a red flag. One lab might be verifying properly. The other isn’t.
• They don’t use reference materials - You can’t check accuracy without them. If they say “we just run the kit,” they’re not verifying.

A 2022 study in Clinical Chemistry and Laboratory Medicine found that labs following ISO 15189 standards had 47% fewer medication errors. That’s not a small difference. That’s the difference between a patient recovering and a patient ending up in the ICU.

What Happens When Labs Get It Wrong

The consequences aren’t theoretical. In 2021, the FDA’s MAUDE database recorded a case where a lab’s digoxin test didn’t account for antibody interference. Twenty-three patients received incorrect doses. One died. Another needed emergency dialysis.

In another case, a transplant center used a tacrolimus test that hadn’t been verified for cross-reactivity with metabolites. They thought their patient’s level was safe. It wasn’t. The patient rejected the new kidney. The lab had skipped testing for 12 key metabolites because “the manufacturer said it was fine.” The manufacturer’s claims were incomplete. The lab didn’t verify.

These aren’t rare. The Institute of Medicine estimates 7,000-9,000 deaths per year in the U.S. are caused by medication errors. A significant portion of those come from inaccurate monitoring.

What You Can Do Today

You don’t need to run a lab. But you do need to protect your patients. Here’s what you can do right now:

• Ask your lab for their verification documentation - Not just a phone call. A written summary. If they refuse, find a new lab.
• Compare results across labs - If you’re sending the same patient to two different labs and getting different numbers, investigate why.
• Use EHR alerts - Many hospitals now have systems that flag when a drug level is outside the therapeutic range. But if the lab’s test is flawed, the alert is useless. Make sure the lab is verified first.
• Train your team - Nurses, pharmacists, and PAs should understand that a lab result isn’t gospel. It’s only as good as the process behind it.
• Support labs that do it right - If you know a lab that follows APHL or CLSI guidelines, keep using them. Reward quality.

The bottom line: your job isn’t done when you order the test. Your job is done when you know the result is accurate. That means asking questions. That means holding labs accountable. That means refusing to accept results from labs that won’t prove their tests work.

What’s Changing in 2025

The FDA is tightening rules. The VALID Act proposal would require all lab-developed tests for medication monitoring to meet the same standards as commercial kits. That’s good news. It means fewer labs will be able to cut corners.

Also, more labs are moving toward integrated EHR systems that auto-flag low or high levels. But those systems only work if the underlying test is verified. A flawed test with a fancy alert is still a flawed test.

The trend is clear: precision medicine depends on precision labs. If you’re prescribing complex medications, you can’t afford to guess. You need proof.