Monitoring During Pembrolizumab: What Tests Will I Need?

Monitoring during pembrolizumab treatment serves two distinct purposes: detecting immune-related side effects before they become serious, and assessing whether the treatment is working. Here is a clear breakdown of what is typically monitored, how often, and why each test matters.

Routine blood tests

Blood tests are the cornerstone of pembrolizumab monitoring. They are typically scheduled before each treatment cycle - or at defined intervals if you are on the six-weekly schedule - and reviewed by your oncologist before the next dose is given. The standard panel includes:

• Full blood count (FBC): Checks red cells, white cells, and platelets. Anaemia is common during cancer treatment; immune-related effects on blood cell counts are less common but can occur.
• Liver function tests (LFTs): Elevated liver enzymes can be an early sign of immune-mediated hepatitis, which is typically asymptomatic in early stages but requires prompt management to prevent progression.
• Thyroid function tests (TFTs): Thyroid dysfunction - both overactive and underactive - is one of the most common immune-related adverse events with pembrolizumab. It is usually detected on routine blood tests before causing symptoms. Most patients require thyroid hormone replacement when this occurs, and it does not usually require stopping pembrolizumab.
• Renal function and electrolytes: Immune-mediated kidney inflammation (nephritis) is uncommon but can occur. Routine monitoring allows early detection.
• Glucose: Immune-mediated diabetes (insulitis) is rare but can present as new hyperglycaemia. Checking glucose at regular intervals is standard practice.

Clinical assessments

At each nurse visit, baseline observations are taken: blood pressure, pulse, temperature, and oxygen saturation where relevant. The nurse will also conduct a structured symptom review - asking about new or worsening symptoms since the last cycle. This conversation is not incidental; it is a core part of the monitoring process.

Between visits, remote check-ins - by telephone or digital assessment - give patients an opportunity to flag new concerns before they escalate. Patients are encouraged to contact the clinical team between scheduled contacts if they develop symptoms that concern them.

Imaging

Response assessment - measuring whether the tumour is shrinking or stable - is done through imaging, typically CT scanning. Imaging is usually performed at defined intervals, such as every 9-12 weeks in the early phase of treatment, and then less frequently as treatment continues. The timing and imaging modality used will depend on your cancer type and clinical situation.

Pembrolizumab can occasionally cause what is called pseudo-progression: an initial apparent increase in tumour size on imaging before a true response occurs, due to inflammation within the tumour. This is why imaging results are interpreted in the clinical context of your overall picture - how you are feeling, your blood test trends, and your cancer type - rather than in isolation.