Question 1

What is the forced swim test and how is immobility scored?

Accepted Answer

The forced swim test (FST), also known as the Porsolt test, is a widely used behavioral assay for depression-like behavior in rodents. An animal is placed in an inescapable cylinder of water for a fixed duration (typically 6 minutes, with the first 1-2 minutes excluded as habituation). The primary behavioral measure is immobility — defined as the absence of active escape movements, with the animal floating passively or making only small movements necessary to keep its head above water. Immobility is interpreted as a behavioral despair or passive coping strategy. Percent immobility equals total immobility time divided by scored session duration, multiplied by 100.

Question 2

What is the difference between immobility, swimming, and climbing in the FST?

Accepted Answer

The modified FST (Detke et al., 1995) distinguishes three behaviors: immobility (floating passively with no active movements beyond those needed to stay afloat), swimming (active horizontal movements across the water surface, often circling the cylinder), and climbing (vigorous upward-directed movements with forepaws against the cylinder wall, often splashing). This three-behavior scoring provides more pharmacological sensitivity than immobility alone — serotonergic antidepressants (SSRIs) preferentially increase swimming, while noradrenergic antidepressants preferentially increase climbing. The sum of immobility + swimming + climbing should equal the total scored session duration.

Question 3

Why is the first 1-2 minutes of the FST typically excluded from scoring?

Accepted Answer

During the first 1-2 minutes of a 6-minute FST session, most animals display vigorous escape behavior (swimming and climbing) regardless of treatment. Including this period inflates swimming/climbing scores equally across groups and reduces the signal-to-noise ratio for detecting drug or genotype effects on immobility. The standard approach scores only the last 4 minutes (minutes 2-6) of the 6-minute session, when inter-individual differences in immobility become more apparent. Some protocols use a 5-minute test and score the full duration. This calculator uses a default 300-second (5-minute) session but allows you to adjust the duration to match your protocol.

Question 4

How is time-bin scoring performed in the FST?

Accepted Answer

In time-bin scoring, the scored session is divided into consecutive 5-second intervals. During each interval, the observer (or automated system) records the predominant behavior: immobility, swimming, or climbing. This yields a temporal profile showing how behavior changes across the session. Time-bin analysis reveals whether treatment effects emerge early or late in the session, whether immobility increases progressively (as expected), and whether any group shows an unusual temporal pattern. The number of 5-second bins scored as immobility, divided by total bins, gives an alternative percent immobility estimate that closely correlates with continuous time scoring.

Question 5

What is latency to first immobility and why is it reported?

Accepted Answer

Latency to first immobility is the time (in seconds) from the start of the scored session until the animal first becomes immobile. It reflects how quickly the animal transitions from active coping (escape attempts) to passive coping (floating). Antidepressant treatments typically increase latency to first immobility — treated animals persist in active escape behavior longer before becoming immobile. Latency is a complementary measure to percent immobility: two animals may show the same total immobility but differ substantially in when they first become immobile, indicating different behavioral dynamics.

Question 6

What are common antidepressant positive controls for the FST?

Accepted Answer

Imipramine (a tricyclic antidepressant, 15-30 mg/kg IP, 30-60 min before test) is the classic positive control and reliably reduces immobility while increasing climbing. Fluoxetine (an SSRI, 10-20 mg/kg IP, often given in 3 doses over 24 hours before test) reduces immobility and increases swimming. Desipramine (a noradrenergic reuptake inhibitor, 10-20 mg/kg IP) reduces immobility and preferentially increases climbing. These pharmacological profiles — swimming vs. climbing — were the basis for the modified FST scoring system. Vehicle-treated animals typically show 60-80% immobility during the scored period, while antidepressant-treated animals show 30-50% immobility.

Question 7

How should I report FST results in a manuscript?

Accepted Answer

Report: (1) species, strain, sex, age, and weight; (2) cylinder dimensions and water temperature; (3) total session duration and scored interval; (4) behavioral definitions for immobility, swimming, and climbing; (5) whether scoring was manual or automated, and if manual, inter-rater reliability; (6) group mean percent immobility with SEM and sample size; (7) latency to first immobility; (8) behavior proportions (immobility/swimming/climbing); (9) statistical test used and exact p-values. If using time-bin scoring, report bin size and show the time course. Include individual data points in figures when possible.

Forced Swim Test Immobility Calculator

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