Sweep Test

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Student MRCS FRCS Fellow

For Medical Students

What It Tests

The sweep test moves fluid from one side of the joint to the other with the hand and reads the result, either as a visible bulge or as a palpable fullness.

It is the same principle as the bulge sign, performed as a continuous sweep around the joint rather than as two separate strokes, and the two names are used for overlapping techniques. 4.8.3 covers the bulge sign.

Its value is that it detects a small effusion, which the patellar tap does not. 4.8.4 covers the tap.

How It Is Performed

Lay the patient supine with the knee extended and the quadriceps relaxed.

Place a hand below the medial joint line and sweep upwards along the medial side and across the suprapatellar region, carrying any fluid with it.

Continue the sweep down the lateral side, so the fluid has been moved from the medial compartment round to the lateral.

Watch and feel the medial side as you release, looking for the fluid returning as a visible bulge or a palpable fullness.

Repeat in the opposite direction, since a knee that produces a finding in one direction and not the other is worth checking twice.

What a Positive Means

A visible or palpable return of fluid medially after a sweep indicates a small effusion.

A palpable fullness without a visible bulge is a positive result in a knee whose contours are hidden, which is why the test is read by hand as well as by eye. 4.23.4 covers those patients.

A negative sweep with a positive patellar tap indicates a larger effusion that is already filling both sides, so nothing moves. 4.8.4 covers the tap.

A negative sweep and a negative tap in a knee the patient describes as swelling means the swelling is extra-articular or intermittent. 4.2.3 covers swelling as a symptom.

Nothing moving in a tense swollen knee indicates a very tense effusion, which is a different finding. 4.8.6 covers ballottement.

Key Learning Points

The test moves fluid from one side of the joint to the other and reads the result.
It shares its principle with the bulge sign and detects a small effusion.
A palpable fullness without a visible bulge is a positive result.
A negative sweep with a positive tap means a larger effusion that is already filling both sides.
Nothing moving in a tense swollen knee indicates a very tense effusion.

MRCS Level

How It Is Performed

In a viva the sweep and the bulge sign are close enough that the useful answer describes the principle and names both.

The expected points are the extended relaxed knee, the continuous sweep from medial to suprapatellar to lateral, the release, and reading the medial side by eye and by hand.

Say that you would read the result by hand as well as by eye, since a palpable fullness is the finding in a knee whose contours are hidden.

Say that you would repeat in the other direction, which takes seconds and improves the yield.

Say that the test is for small effusions, and name the test for large ones. 4.8.6 covers ballottement.

What a Positive Means

The reasoning expected is that a small volume of fluid can be displaced within a single capsular space, so moving it and watching it return is the only way to detect it clinically. 4.8.1 covers normal fluid physiology.

The suprapatellar pouch is the route the fluid takes between the two sides, which is why the sweep passes across it rather than straight across the joint line.

A larger volume already occupies both sides, so there is nowhere for it to move to, which is why the sweep fails at larger volumes and the tap succeeds. 4.8.4 covers the tap.

The parapatellar hollows are where a returning volume becomes visible, which is why the reading is taken there. 4.8.3 covers the bulge sign.

How Good It Is

The sweep is among the more sensitive effusion tests and is read both visually and by palpation, which gives it an advantage in a knee whose contours are hidden. 4.23.4 covers those patients.

Its reproducibility is moderate, as with all the small effusion tests, and better for the palpable than for the visual reading.

It fails at large volumes for a mechanical reason rather than a technical one, which is why more than one test is performed. 4.8.4 covers the tap.

It cannot say what the fluid is, which no clinical test can. 4.8.7 covers the acute haemarthrosis.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

Key Learning Points

A small volume can be displaced within a single capsular space, so moving it is how it is detected.
The suprapatellar pouch is the route the fluid takes between the two sides.
A larger volume already occupies both sides, so there is nowhere to move it to.
The parapatellar hollows are where a returning volume becomes visible.
It fails at large volumes for a mechanical reason rather than a technical one.

FRCS Level

How Good It Is

At decision level the sweep contributes exactly what the bulge sign does, which is detecting the effusion that would otherwise be recorded as absent.

Its practical advantage is being readable by hand, which makes it the better test in an obese knee where the visual reading is unavailable. 4.23.4 covers those patients.

Recording a small effusion changes the interpretation of the range and the quadriceps findings taken at the same visit. 4.8.2 covers the significance.

A persistent small effusion detected only by this test is the finding that moves a patient towards an inflammatory assessment, and the move is consequential. 4.8.9 covers synovitis.

In the post-operative knee it explains a stalled range and a persisting lag at a volume the other tests miss. 4.7.10 covers post-operative range.

Where It Misleads

The commonest error is performing only a patellar tap and concluding a dry knee. 4.8.4 covers it.

The second is reading only by eye, which discards the test's advantage.

The third is performing the effusion tests after the painful parts of the examination.

The fourth is treating a negative sweep in a tense knee as a dry knee, when nothing can move. 4.8.6 covers ballottement.

The fifth is recording an effusion without a size. 4.1.8 covers the record.

Key Learning Points

The sweep detects the effusion that would otherwise be recorded as absent.
Being readable by hand makes it the better test in an obese knee.
Recording a small effusion changes the interpretation of the range and the quadriceps.
A persistent small effusion moves a patient towards an inflammatory assessment.
A negative sweep in a tense knee is not a dry knee.

Fellowship Level

Where It Misleads

In referred practice this test and the bulge sign are the two that change the record from a dry knee to an effused one, and that change is frequently the answer.

A patient with a persistent small effusion through months of rehabilitation has an irritated joint, and the programme cannot succeed until the cause is addressed. 4.3.9 covers wasting.

A patient in whom only the tap was ever performed has a record of dry knees that cannot be relied on, and repeating the assessment is the first step rather than further imaging. 4.8.4 covers the tap.

A patient with a chronically effused knee and a popliteal cyst has a joint producing fluid, and the cyst's behaviour follows the joint. 4.6.10 covers the popliteal fossa.

In a knee with a scarred or partially obliterated suprapatellar pouch the fluid cannot be swept across, so a negative result is less informative and the previous surgery explains it. 4.2.30 covers previous surgery.

In medicolegal work the distinction between an effusion that was absent and one that was not looked for is rarely recoverable from a record, and this test is usually the one that was not performed. 4.23.7 covers that setting.

Key Learning Points

This test and the bulge sign change the record from a dry knee to an effused one.
A persistent small effusion through rehabilitation means the programme cannot succeed yet.
A record of dry knees based on the tap alone cannot be relied on.
A scarred suprapatellar pouch makes a negative result less informative.
The distinction between absent and not looked for is rarely recoverable from a record.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 023 rather than an omission. The comparative standing of these tests is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page, and chapter 3.17 lists the studies the figures would come from. Any measurement here, including a flexion angle, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.