A therapist holding a compression garment during a fitting at Renu

Lymphedema screening: bioimpedance vs tape measure

A tape measure tracks limb volume (5 to 10% thresholds); bioimpedance reads extracellular fluid (L-Dex). What the PREVENT trial found.

By The Renu lymphatic and massage team · 8 min read

Tape measurements and bioimpedance answer the same question two different ways. A tape measure records girths along the limb and converts them to volume, with a 5 to 10% relative increase over your own baseline counted as subclinical lymphedema and 10% or more as clinical. Bioimpedance spectroscopy passes a spectrum of electrical frequencies through the limb to estimate extracellular fluid, reported as an L-Dex score. In the PREVENT trial, bioimpedance triggered fewer and later early interventions and far fewer people went on to chronic lymphedema (7.9% versus 19.2%). The NLN insists that any number sits beside symptoms and a clinical examination.

Two ways of asking one question

Screening for lymphedema means catching a change in a limb before you can see it. The two methods most people meet do this differently. A tape measure asks “how big is the arm, compared with before and compared with the other side”. Bioimpedance asks “how much fluid is sitting outside the cells in this arm, compared with the other side”. Size changes with fat and muscle as well as fluid; fluid changes before size does. That difference explains most of what follows.

The tape measure, and the numbers it produces

The NLN’s 2025 diagnosis statement describes the method: circumferences are taken with a flexible tape at regular intervals along the limb (for example every 4 cm or 10 cm) or at anatomical landmarks, then converted to a volume using the truncated cone formula. It is portable and inexpensive, its reliability improves with a standard protocol and the same measurer each time, and interval-based protocols are not interchangeable with other methods, so a 4 cm series cannot be compared with a 10 cm one.

Three rules from the NLN turn those girths into a diagnosis:

Measure both limbs. Lymphedema is one-sided; weight change is two-sided. The unaffected arm itself varied by a median of 10.5% within the same person across repeated measurements in one screening cohort, which is why a single limb measured on its own tells you little.
Prefer relative to absolute change. Older cut-offs such as a 2 cm difference or a 200 mL volume difference (both still cited by the National Cancer Institute as thresholds some experts use) shift with body weight and BMI. Relative volume change, the percentage change in the at-risk arm corrected for change in the other arm, is independent of those and is the preferred metric.
Use thresholds against your own baseline. A relative increase of 5 to under 10% is subclinical lymphedema; 10% or more is clinical. Without a pre-operative baseline, the NLN warns, misdiagnosis rates can reach 60%, because arms are commonly asymmetrical before surgery and early post-operative measurements are unreliable as a starting point.

The tape measure also has a track record in early intervention. Stout Gergich’s 2008 study measured 196 women before surgery and every three months after it, prescribed a four-week compression sleeve whenever the arm grew by more than 3%, and brought the swelling down by a mean of 48 mL with the reduction holding at follow-up. The tape measure is not the old-fashioned option; it is the method most of the early-detection evidence was built on.

Bioimpedance: reading the fluid, not the size

Bioimpedance spectroscopy (BIS) passes a range of electrical frequencies through the limb. Fluid outside the cells conducts differently from fluid inside them, so the device can estimate extracellular fluid, which is what accumulates first in lymphedema. The result for an arm is usually reported as an L-Dex score, comparing the at-risk limb with the other one.

The NLN summarises the performance data. Against volume-based methods, L-Dex showed better discrimination in detecting breast cancer-related lymphedema (area under the curve 0.832 versus 0.649). Devices have a high positive predictive value (96.7%) but moderate sensitivity (72.5%) and specificity (87.5%), and limited accuracy in distinguishing fluid from fat. Results vary between devices, so each needs its own reference ranges, and the equipment takes space and money. In early-stage disease, the NLN says BIS may aid detection and reduce progression to full decongestive therapy.

The PREVENT trial

The study that moved the field was published by Ridner and colleagues in 2022. It enrolled 1,200 new breast cancer patients across international sites, randomised 963 to prospective surveillance by either BIS or tape measurement, and analysed 879. All had risk factors: mastectomy or partial mastectomy with axillary dissection, a sentinel biopsy of more than six nodes or axillary radiation, chest wall or supraclavicular radiation, or taxane chemotherapy.

Both arms used the same early intervention when a trigger was met: a class 2 (23 to 32 mmHg) compression sleeve and gauntlet worn 12 hours a day for four weeks. The tape-measure trigger was a volume increase of 5% to under 10% over baseline and the other arm. The BIS trigger began at a 10-unit L-Dex rise and was lowered to 6.5 units during the trial as new data appeared. In either group, a 10% volume change meant referral for complete decongestive therapy, which was the definition of chronic lymphedema.

Outcome (median follow-up 32.9 months)	Bioimpedance	Tape measure
Triggered an early intervention	20.1%	27.5%
Median months to trigger	9.7	3.9
Progressed to chronic lymphedema after intervention	7.9%	19.2%

The relative risk of progression with BIS was 0.41, an absolute reduction of 11.3 percentage points. The authors’ reading is that BIS identifies more precisely who will benefit from a short course of compression: fewer people were put into sleeves, they were put in later, and fewer went on to chronic swelling. The tape-measure group, triggered earlier by a size change that could reflect many things, had more false alarms and, it seems, more missed real ones.

Why the evidence looked different in 2016

Six years earlier, a review by Seward and colleagues of research going back to 1992 concluded that bioimpedance was an accurate diagnostic tool for existing lymphedema but had not been validated for early detection. That was a fair reading of the data at the time, and it is still quoted. The PREVENT trial is the large randomised study that was missing then. If your surgical team or clinic sounds more cautious about BIS than this page, they may be working from the earlier literature; if they sound more enthusiastic, remember the sensitivity figure of 72.5%, which means roughly one in four early cases is missed by the device alone.

A number needs a person beside it

Both NLN statements make the same point. The 2026 risk-reduction statement says best practice incorporates objective measurements with symptom report and clinical examination for an accurate diagnosis, rather than relying on measurements alone. The diagnosis statement adds that volume measures cannot distinguish lymphedema from other kinds of swelling, and that combining risk factors, symptoms and examination improves the picture. Heaviness, a sense that the arm is bigger, or a perception of swelling are symptoms the NLN wants reported immediately, whatever the last measurement said.

The screening schedule is the other half. The NLN suggests people at high risk (after axillary dissection) be measured as often as every three months for the first two years and then less often through years three to five, and people at lower risk about every six months, combined with self-monitoring. Which device is used matters less than that a baseline exists and the measurements continue. Our page on baseline arm measurements before breast surgery explains why the first one matters most, and early signs of arm lymphedema covers the symptoms to report between visits.

What this means for you in Calgary

If your cancer programme offers bioimpedance surveillance, take it, keep every appointment, and still tell someone about a tight ring or a heavy arm. If it does not, tape-measure surveillance with a pre-operative baseline is well supported and is what most of the early-intervention literature used.

Bioimpedance is a hospital and specialist-clinic tool; ask your cancer centre whether it is part of your follow-up. In a lymphedema clinic the usual approach is the one the NLN describes: tape measurement of both limbs, a record of your symptoms and an examination of the tissue, read together. Meeghan Mackenzie, our Certified Lymphedema Therapist, is the person to ask about a baseline and follow-up measurements at Renu. If a measurement crosses the subclinical threshold, the response is the one the trials used: a short course of compression, fitted properly (a fitting is 30 minutes, $25), with lymphatic drainage where it helps, and a return to monitoring. Established lymphedema moves to complete decongestive therapy. Our Learning Centre page on lymphedema and lipedema explains the stages those thresholds map onto.

When to call your oncology team

A screening number is never the reason to delay a call. If the limb becomes red, hot, painful or suddenly larger, or you have a fever or chills, the NLN treats that as a suspected infection and an urgent medical event, because cellulitis causes further lymphatic damage. Sudden one-sided swelling with pain also needs a clot excluded. Either of those goes to your team the same day; a slow, painless change in size or sensation goes to whoever runs your surveillance within days, and to us if you would like a second pair of trained hands.

Questions people ask

What does an L-Dex score mean?

L-Dex is the lymphedema index produced by a bioimpedance device, comparing the extracellular fluid in the at-risk limb with the other limb. In the PREVENT trial, a rise of 6.5 or more L-Dex units above a person's own baseline, without a 10% volume change, triggered a four-week compression intervention; the trial had started with a 10-unit trigger and lowered it part-way through. Reference ranges differ between devices, so a score is read against the machine that produced it.

Is a 2 cm difference between my arms lymphedema?

Not on its own. The NCI notes that some experts treat a 2 cm difference at any point, or a 200 mL volume difference, as clinically significant, but the NLN now prefers relative volume change over absolute cut-offs because 2 cm or 200 mL shifts with body weight and BMI. A 5 to under 10% increase over your pre-operative baseline is the subclinical threshold, 10% or more is clinical, and arms are often asymmetrical before surgery.

Is bioimpedance more accurate than a tape measure?

It is more sensitive to early fluid change but not infallible. The NLN's diagnosis statement reports an area under the curve of 0.832 for L-Dex against 0.649 for volume methods, with a positive predictive value of 96.7% but sensitivity of 72.5% and specificity of 87.5%, and limited ability to tell fluid from fat. A 2016 review concluded it was accurate for existing lymphedema but not yet validated for early detection; the PREVENT trial, published in 2022, is the evidence that has moved since.

Does Renu use bioimpedance?

Bioimpedance devices are found in hospitals and specialist clinics rather than massage clinics. Ask your cancer centre whether it is part of your follow-up. At Renu, Meeghan Mackenzie, our Certified Lymphedema Therapist, is the person to ask about tape-measure baselines and follow-up, which is the common clinic approach the NLN describes.