Background: Early detection of suicidal behaviour is important for patient recovery and quality of life.Detecting people in acute crisis is also important.Suicide is a rare event and the aetiology is multifaceted, and many current clinical predictors and markers are not specific for suicide, except a previous suicide attempt.Any given suicidality test must be highly sensitive (no false negatives) to avoid missing suicidal patients.Method: A systematic literature search was performed in Scopus, Pubmed, and Web of Science databases.Search terms used were title: suicid*, test*, all fields: biolog*.Titles and abstracts of the articles found were scanned and duplicates eliminated.Relevant articles were obtained and a synthesis made.Results: Altogether 26 articles were obtained after screening for relevance and duplicates from the searches.The structure of the brain may be altered in depression and suicide, and MRI may reveal abnormalities such as white matter or gray matter hyperintensities.Measuring blood cortisol (the human stress hormone) for suicide "testing" has also been suggested, as abnormal immunological function appears to be present in suicidal conditions.However, cortisol is not specific to suicide.The dexamethasone suppression test (DST) measures glucocorticoid receptor mediated negative feedback of the HPA axis, and may be predictive of future suicidal behaviour.If the DST response is abnormal, the HPA axis does not function properly.The DST does not seem to function universally in all patient settings, as some studies have failed to seeany differences between suicide victims and controls while others have found non-suppression in the DST to be associated with higher suicide risk.Another functional test can be done with D-fenfluramine, which releases serotonin into synaptic clefts and inhibits its reuptake, increasing prolactin hormone level.If serotonin function is impaired as in e.g.suicidal states, a lower prolactin response is observed.Peripheral serotonin activity can be used as a marker of central activity, and tritiated imipramine binding to platelet serotonin transporters can be used to detect patients with low serotonin activity and possible suicidal intent.Plasticity enhancer BDNF can also be measured from blood samples, but so far little is known how well these levels describe cerebral concentrations.Measurements of 5-HIAA, HVA, and MHPG, metabolites of serotonin, dopamine, and noradrenaline, respectively, can be used to investigate central levels of these neurotransmitters and to estimate suicide risk at least in bipolar patients.There are still little results available about genetic markers, and studies have been fairly small scale.Conclusion: No currently available single biological marker alone can reliably predict suicidality, since it is such a complex phenomenon, with several factors bringing about the outcome of suicide.Combining results from several biological markers would potentially improve sensitivity, but also increase the costs of testing and the potential for errors.As more research is being done on genetic markers of suicidality, more comprehensive tests of co-expression of several genes may be investigated.