DOMAIN 1: Patient Selection
A: Risk of Bias
1. Was a consecutive or random sample of patients enrolled? Yes/No/Unclear
2. Did the study avoid inappropriate exclusions?
	Yes	Patients are included in follow‐up post radical CRC resection, OR Exclusions was justified in the text and reviewers reached consensus on the appropriateness of any exclusions. Exclusions based on patient characteristics allowing subgroup analysis (e.g. tumour grade) should be deemed appropriate
	No	Criteria for “yes” not achieved.
	Unclear	Exclusions not reported clearly.
OVERALL RISK OF BIAS: LOW/HIGH/UNCLEAR
B: Applicability
1. Is there concern that the included patients do not match the review question?
	Yes	Patients are not undergoing follow‐up post radical CRC resection including CEA measurement.
	No	Patients are undergoing follow‐up post radical CRC resection including CEA measurement.
	Unclear	The included population is not defined.
OVERALL CONCERN REGARDING APPLICABILITY: LOW/HIGH/UNCLEAR
DOMAIN 2: Index Tests
A: Risk of Bias
1. If a threshold was used, was it pre‐specified? Yes/No/Unclear
2. Is the same method and instrument used for all CEA measurements? Yes/No/Unclear
3. Is there an estimation of reproducibility of the method, for example the % coefficient of variation at specific concentrations? Yes/No/Unclear
4. Is there an indication of method accuracy, for example, is there evidence of participation in an external quality assessment and proficiency testing scheme? Yes/No/Unclear
OVERALL RISK OF BIAS: LOW/HIGH/UNCLEAR
B: Applicability
1. Is there concern that the index test, its conduct, or interpretation differ from the review question?
	Yes	Blood CEA is not interpreted as a stand‐alone test to trigger investigation for CRC recurrence
	No	Blood CEA is interpreted as a stand‐alone test to trigger investigation for CRC recurrence
	Unclear	It is unclear whether the index test differs from the review question
OVERALL CONCERN REGARDING APPLICABILITY: LOW/HIGH/UNCLEAR
DOMAIN 3: Reference Standard
A: Risk of Bias
1. Is the reference standard likely to correctly classify the target condition? ‐ can we confidently exclude recurrence on the basis of no clinical detection of recurrence when we are assessing the utility of CEA at detecting asymptomatic recurrence amenable to resection?
	Yes	An appropriate reference standard (as defined in the protocol) is used.
	No	An inappropriate reference standard is used
	Unclear	The reference standard used is not clearly specified.
2. Were the reference standard results interpreted without knowledge of the results of the index test? ‐ If tests are done as part of a follow‐up regime it must not be assumed that the interpretation of each test is independent of another. It must be clearly stated when reference test interpretation occurred.
	Yes	The reference standard results were interpreted without knowledge of the index test(s).
	No	The reference standard results were interpreted with knowledge of the index test(s).
	Unclear	It is not clear whether interpretation was blinded or not.
OVERALL RISK OF BIAS: LOW/HIGH/UNCLEAR
B: Applicability
1. Is there concern that the target condition as defined by the reference standard does not match the review question? Yes/No/Unclear
OVERALL CONCERN REGARDING APPLICABILITY: LOW/HIGH/UNCLEAR
DOMAIN 4: Flow and Timing
A: Risk of Bias
1. Was the index test repeated prior to the reference standard? Yes/No/Unclear
2. Was the the timing between index test(s) and reference standard ascertainable?
	Yes	The timing was ascertainable.
	Unclear	Not reported, variable or could not be clearly determined
3. Did all included patients who had at least one CEA measurement receive a reference standard? Yes/No/Unclear
4. Did patients receive the same reference standard?
	Yes	>95% of patients received the same reference standard regardless of index test results or place within a follow‐up schedule.
	No	>95% of patients did not receive the same reference standard regardless of index test results, or place within the follow‐up schedule.
	Unclear	It is unclear whether all the included patients received same reference standard regardless of index test results
5. Were all patients included in the analysis? Yes/No/Unclear
OVERALL RISK OF BIAS: LOW/HIGH/UNCLEAR