Signs That Keytruda Is Working: Test Results and More

Friendly heads-up: This article is for general education and not medical advice. Only your oncology team can interpret your scans, labs, symptoms, and overall plan. If anything feels suddenly worse (especially breathing, severe diarrhea, chest pain, confusion, or yellowing eyes/skin), contact your care team right away.

Why “Is Keytruda working?” isn’t a yes/no question

Keytruda (pembrolizumab) is an immunotherapy drugspecifically a PD-1 checkpoint inhibitor. Translation: it doesn’t attack cancer cells like a tiny chemo ninja. It “takes the brakes off” parts of your immune system, so your own T cells can recognize and fight cancer better.

That’s also why Keytruda can feel like the world’s most high-stakes group project: your immune system is doing the work, which can mean responses look different than chemo or radiation. Some people see clear improvement quickly. Others need weeks or months before scans show change. And sometimes the first scan is… confusing. (Medical term: “equivocal.” Real-life term: “Are you kidding me?”)

The #1 sign Keytruda is working: imaging results trending the right way

If you remember only one thing, make it this: CT and MRI scans are usually the main way doctors judge whether Keytruda is working. Your team compares your current scan to your baseline scan and looks for patterns over time.

What doctors look for on CT/MRI

• Tumors shrinking (often called a “partial response”)
• Tumors disappearing (a “complete response” on imaging)
• No meaningful growth (called “stable disease,” and yesstable can be good)
• New lesions or clear growth (suggesting “progressive disease”)

What “stable disease” can mean (and why it’s not a consolation prize)

With immunotherapy, “stable” may still be a winespecially if you’re feeling better, labs are acceptable, and the overall tumor burden isn’t climbing. In many cancers, the goal isn’t only shrinkage; it’s also durable control. Some patients stay stable for a long time and live well while the immune system keeps doing its job.

PET scans: sometimes helpful, not always required

Depending on your cancer type, your doctor may use PET/CT (or other imaging) to look at metabolic activitybasically, whether spots are “lighting up” less over time. PET can be useful in certain situations, but CT/MRI measurements remain the standard approach for many treatment decisions. The key is trend + context: one scan rarely tells the whole story.

When scans look worse at first: pseudoprogression (and other curveballs)

Immunotherapy can trigger inflammation inside or around tumors as immune cells move in. That can temporarily make a tumor look bigger on imagingor make new tiny spots easier to see. This is one reason your team may want repeat scans before declaring treatment failure (as long as you’re clinically stable).

Pseudoprogression: “It got bigger… then it got better”

Pseudoprogression is an initial increase in apparent tumor size (or new lesions) followed by shrinkage or stability. The good news: it’s real. The less-fun news: it’s not super common overall. In large analyses across cancers and checkpoint inhibitors, pseudoprogression has been estimated around the single-digit percentage range. Still, it’s important enough that doctors actively consider itespecially when you’re otherwise doing okay.

iRECIST: why doctors sometimes “wait to confirm” progression

Because pseudoprogression can happen, researchers developed immune-adapted response rules (often referred to as iRECIST) that may require confirming progression on a later scan before changing courseprovided the patient is not clinically deteriorating. In plain language: if you’re stable and the scan is borderline, your doctor may say, “Let’s re-scan and confirm before we pivot.”

Not every “worse scan” is pseudoprogression

There are other patterns your team considers:

• Delayed response: the tumor doesn’t shrink right away, but later scans improve.
• Mixed response: some tumors shrink while others grow, which can lead to targeted radiation or other adjustments.
• True progression: consistent growth plus worsening symptoms.
• Rare rapid progression: a small subset of patients may worsen quickly on immunotherapy, and doctors take this seriously.

Test results that matter: blood work, labs, and “numbers”

Scans tell you what the cancer is doing. Labs often tell you what your body is doingboth in terms of safety and sometimes clues about response.

Routine monitoring labs: the “safety dashboard”

Most patients on Keytruda have periodic labs that can include:

• Liver enzymes (to watch for hepatitis or liver irritation)
• Kidney function (creatinine and related markers)
• Thyroid function (hypothyroidism/hyperthyroidism can occur)
• Blood counts and other basics, depending on your regimen and diagnosis

Important nuance: abnormal labs can sometimes reflect immune-related side effects rather than “proof” the drug is working against cancer. They’re still meaningfuljust in a different way. Think of labs as the check-engine light and temperature gauge, not the lap-time scoreboard.

Tumor markers: helpful for some cancers, irrelevant for others

Tumor markers are blood (or other fluid) tests that can be useful in specific contextslike PSA in prostate cancer, CA-125 in some ovarian cancers, CEA in certain colorectal cancers, and others. If your cancer has a marker that reliably tracked disease before treatment, your team may follow it during Keytruda.

But tumor markers have limitations:

• Not every cancer has a useful marker.
• Markers can fluctuate due to inflammation, infection, or other factors.
• Trends matter more than single values.
• Markers can lag behind imaging (or sometimes move before imaging does).

If your marker is dropping over multiple checks and scans are stable or improving, that’s usually reassuring. If markers rise but scans improve, your oncologist may prioritize imaging and symptoms over a single lab value.

Biomarkers: more about “likelihood” than “live tracking”

You may hear about biomarkers such as PD-L1 expression, MSI-H/dMMR status, tumor mutational burden (TMB), and others. These can help predict who may benefit from immunotherapy or guide whether Keytruda is an option. They’re typically not used as a day-to-day “is it working this week?” meter.

Symptoms that can suggest improvement (with a big asterisk)

Many people want a simple sign: “I feel better, so it must be working.” Sometimes that’s true! But symptoms can be influenced by many things (stress, sleep, anemia, thyroid changes, infections, other meds). The most helpful approach is to track patterns.

Common “better” trends patients notice

• Less cancer-related pain or less need for pain medicine
• Improved breathing or less cough (for lung involvement)
• More appetite and steadier weight
• Improved energy and ability to do normal activities
• Fewer “pressure” symptoms (less swelling, less fullness, less neurologic irritation depending on tumor location)

These improvements can be meaningful, especially when they match imaging. But symptom improvement alone isn’t proofbecause sometimes supportive care (steroids, fluids, pain control, anti-nausea meds, radiation to a painful spot) improves quality of life while the cancer picture stays the same.

Side effects: sometimes a clue, never a trophy

Keytruda can cause side effects like fatigue, rash, diarrhea, cough, appetite changes, or thyroid issues. It can also trigger immune-related side effects in many organs because the immune system may become overactive.

Some patients wonder: “If I’m having side effects, does that mean Keytruda is working?” The honest answer: not reliably. Many people respond beautifully with few or no side effects, and some people have significant side effects without a strong anti-cancer response. Side effects show immune activity, but they don’t confirm the immune system is targeting the tumor effectively.

Red-flag symptoms to report quickly

Call your oncology team urgently (or seek emergency care if severe) for symptoms like:

• New or worsening shortness of breath, chest pain, or persistent cough
• Severe or persistent diarrhea, blood in stool, or strong abdominal pain
• Yellow skin/eyes, dark urine, severe nausea/vomiting, or right-sided belly pain
• Severe headache, vision changes, confusion, fainting, or unusual weakness
• Extreme thirst/urination, very high sugars, or signs of dehydration
• Major rash, blistering, or skin peeling

These can be signs of immune-related inflammation (pneumonitis, colitis, hepatitis, endocrine problems, and more). Early evaluation matters because prompt treatmentoften including steroidscan prevent complications.

A practical checklist: what to ask at your next Keytruda visit

If you want clearer answers (and fewer vague “we’ll watch it” moments), take these questions with you:

• Which lesions are we tracking? (Ask about “target lesions” and baseline measurements.)
• What did my scan show in plain English? (Shrink, stable, mixed, or growthand by how much?)
• Are we concerned about pseudoprogression? If yes, what makes it plausible in my case?
• When is the next scan, and what would change the plan sooner?
• Which labs are we monitoring regularly? (Thyroid, liver, kidney, etc.)
• Do I have a tumor marker that’s meaningful? If yes, what trend would be reassuring?
• What side effects should I report immediately? (Get a short list tailored to you.)
• If Keytruda stops working, what’s Plan B? Knowing options can reduce anxiety.

What it looks like when Keytruda isn’t working

Doctors usually make the “not working” call based on a combination of:

• Consistent tumor growth on repeat imaging
• Worsening symptoms that align with cancer progression
• Declining overall condition (such as performance status)
• Unmanageable toxicity that makes it unsafe to continue

If Keytruda isn’t benefiting you, the next steps might include switching therapies, adding or changing chemotherapy, targeted therapy (if you have a targetable mutation), radiation to specific problem areas, surgery in select situations, or clinical trials. This is highly individualizedand it’s also why your oncologist keeps returning to the “big three”: scans, symptoms, and safety.

Scanxiety is real: how to survive the waiting game

Waiting for results can feel like refreshing a tracking number that never updates. A few coping strategies patients commonly find useful:

• Ask when results will be available and who will call you.
• Keep a running notes app of symptoms and questions so you don’t have to remember everything under stress.
• Bring a second set of ears (friend/family) or request permission to record the visit.
• Focus on what you can control: hydration, nutrition support, gentle movement, sleep, and reporting symptoms early.
• Request clarity: “What is the response category, and what does that mean for the plan?”

Bottom line: the most reliable signs Keytruda is working

In most cases, the strongest “Keytruda is working” signals look like this:

• Scans show shrinkage or stability over time
• You feel better (or at least not worse) in a way that matches imaging
• Any relevant tumor markers trend down or stabilize (when applicable)
• Labs and side effects are manageable and monitored appropriately

And if the first scan is unclear, you’re not alone. Immunotherapy can be slower and stranger than older treatments, which is why repeat imaging and careful clinical judgment are part of the process.

Experiences: what the Keytruda “is it working?” journey often feels like

Note: The experiences below are generalized patterns commonly reported in oncology care settings and patient education conversationsnot personal stories from any one individual. Your experience may be completely different, and that’s normal.

1) The first scan feels like a final exam you didn’t know you were taking. Many people describe the weeks leading up to the first follow-up CT/MRI as mentally exhausting. You might catch yourself scanning your body for “clues” like a detective in a drama seriesexcept the plot twists involve lab portals and radiology wording. A common theme is learning that how you feel and what the scan shows don’t always match perfectly. Some patients feel better before the scan improves. Others feel tired (from treatment or stress) even as tumors shrink. The takeaway many people adopt: don’t judge the whole semester by one quiz.

2) “Stable” becomes a surprisingly beautiful word. People often go into immunotherapy hoping for dramatic shrinkagebecause that’s the movie version of cancer treatment. In real life, stability can be meaningful, especially when it lasts. It’s common to hear patients say that once their oncologist explained what stable disease could mean (control, time, and sometimes a delayed response), it reframed the entire experience. “Stable” stops sounding like “meh” and starts sounding like “we’re holding the line.”

3) Side effects can be confusing emotionally. Some patients feel oddly reassured by mild side effects (“At least something is happening”), while others feel betrayed by them (“I’m paying the priceso where’s the benefit?”). Over time, many learn a more useful mindset: side effects are information, not a scoreboard. A rash might be manageable and treated while you stay on therapy. Thyroid changes might explain fatigue and improve with replacement medication. But serious symptomslike worsening shortness of breath or persistent diarrheatend to shift the emotional tone quickly from “Is it working?” to “Let’s get this checked now,” which is exactly the right move.

4) The “pseudoprogression” conversation is both hopeful and stressful. When a scan looks worse, some patients are told it might be inflammation rather than true growth. That can feel like being offered a life raft made of question marks. People often describe a second waiting periodanother scan, another month, another round of uncertainty. What helps in this stretch is having a clear plan: what symptoms would mean “don’t wait,” what the next scan timing is, and what the fallback options are if progression is confirmed. Clarity doesn’t remove anxiety, but it does make anxiety less chaotic.

5) Many patients become “data organizers” without meaning to. A common practical shift is that patients start tracking a few simple items: energy level, appetite, bowel habits, breathing changes, weight trends, temperature, and any new symptoms. Not obsessivelyjust enough to answer the question, “Has anything changed since last visit?” This helps because immune-related side effects can pop up in unexpected ways, and early reporting is often the difference between a manageable issue and a bigger interruption.

6) The most repeated advice is also the most boringand the most useful: talk to your team early, not heroically late. Patients frequently say they wish they’d reported certain symptoms sooner (especially diarrhea, cough/shortness of breath, or extreme fatigue). The goal isn’t to be “tough.” The goal is to stay safe, stay on an effective plan when possible, and adapt quickly when needed.

If you’re in the middle of this process right now, it’s okay to want certainty. But with Keytruda, progress is often measured in trends, not instant answersand your care team’s job is to interpret those trends with you.

Conclusion

The clearest signs that Keytruda is working usually come from imaging over time (shrinkage or stability), supported by how you’re feeling, any relevant tumor marker trends, and safe lab monitoring. Immunotherapy can look “slow” or “weird” on early scans, which is why follow-up imaging and careful clinical judgment matter. Stay in close contact with your oncology team, report new symptoms early, and remember: “working” doesn’t always mean dramaticit often means durable.