Eosinophil Cationic Protein Unit Converter

Convert ECP between µg/L, ng/mL and pg/mL — the first two are the same number. The reason this page exists is the sentence under the ladder: eosinophils keep releasing ECP into the tube after the blood is drawn, so a serum ECP without a stated clotting time and temperature cannot be compared with anyone’s reference interval, including the one printed beside it.

Last updated 18 Sep 2026

ECP converter

Mass units only

Serum eosinophil cationic protein

µg/L and ng/mL are numerically identical; pg/mL is that number × 1,000. There is no molar unit for ECP and there should not be — see the ladder note below.

Reference figureTwo reference laboratories, the same fluorescent enzyme immunoassay family, upper limits threefold apart: ARUP gives less than 17.8 µg/L and Labcorp gives 4–54 µg/L on ImmunoCAP. Neither is wrong for its own laboratory. The spread is what a reader should take away, because it is larger than most of the differences anybody wants to call clinically meaningful.

12.0µg/LExample

Serum ECP 12 µg/L, clotted 60 minutes at 22 °C

The ladder, and why there is no molar unit

1 µg/L = 1 ng/mL (identical)
1 µg/L = 1,000 pg/mL
no nmol/L or pmol/L is offered

µg/L = ng/mL: one microgram in a litre is one nanogram in a millilitre. The prefix and the volume both change a thousandfold and cancel, so the number does not move — which is why laboratory reports print one or the other with no conversion beside it
pg/mL: a thousand picograms make a nanogram, so pg/mL is the µg/L figure multiplied by 1,000. Uncommon for serum ECP, but it turns up in sputum, nasal lavage and research assays where concentrations are quoted at finer resolution
no molar unit: ECP is a ribonuclease of roughly 18–21 kDa whose mass varies with glycosylation, measured against an assay calibrator rather than against a molar mass. There is no single grams-per-mole to divide by, so a pmol/L figure would be an assumption presented as a conversion
the number that is not a unit problem: the clotting time and temperature. No arithmetic on this page fixes a sample that sat warm for three hours, and none of the units above tell you whether it did

Worked example

Serum ECP 12 µg/L, clotted 60 minutes at 22 °C
12 µg/L is 12.0 ng/mL — the same number, because the two units are identical
12 × 1,000 = 12,000 pg/mL
Inside both of the reference figures quoted above (under 17.8 µg/L on one, inside 4–54 µg/L on the other)
The protocol matters more than the conversion: the same blood clotted for an hour at 37 °C instead of 22 °C would report roughly two to four times this figure
For the measurement guidance actually uses, see the absolute eosinophil count calculator and read the result on the eosinophilia severity interpreter

The pre-analytical protocol, from two laboratories’ own instructions

This is the whole argument of the page. Every other analyte on this site has a pre-analytical section that amounts to "avoid haemolysis and separate promptly"; ECP has a protocol with a stopwatch in it, because the analyte is still being manufactured inside the specimen. A result reported without the clotting time and temperature is not comparable with a reference interval, with a previous result from another laboratory, or with a published study — and the laboratory that measured it is the only party who knows what protocol it used.
Step	What the instructions say	Why it is part of the result
Clotting time	"Release ECP by clotting for 60-120 minutes at room temperature (20-24° C)" (ARUP); "Allow the sample to stand for 60 to 120 minutes to release the ECP in the cells prior to centrifugation" (Labcorp)	Eosinophils degranulate in the tube. The measured ECP is partly what was circulating and partly what the cells released after the needle came out, and the split depends on how long you waited
Temperature during clotting	20–24 °C, stated explicitly by one of the two	A systematic review puts the effect at "two to four times at 37 °C relative to 22 °C when stored for an hour". That is a larger swing than almost any clinical difference the test is asked to detect
Centrifugation	"Centrifuge at 1000-1300 x g for 10 minutes at room temperature" (ARUP)	Separating the serum stops the release. Until it happens, the clock is still running
Serum, not EDTA plasma	Serum is the specified matrix	"The levels of ECP in serum is often five to ten times of those in plasma because eosinophils in blood containing EDTA are inactivated." A plasma ECP and a serum ECP are not interchangeable numbers

Two reference intervals, one assay family, threefold apart

The upper limits differ by a factor of three — 17.8 against 54 µg/L — on what is broadly the same immunoassay. Part of that is genuine method difference and part of it is the protocol variable in the table above, which no reference interval can absorb. The practical consequence is that a serial ECP is only interpretable within one laboratory, and a single ECP compared against a number from anywhere else is not interpretable at all. Use the interval printed beside your own result, and note that the paediatric series in the last row separates treated from untreated children cleanly while sitting inside one of the two published adult intervals and outside the other.
Source	Method	Interval	What the source says with it
ARUP Laboratories, test 3000537	Quantitative fluorescent enzyme immunoassay	"Less than 17.8 ug/L"	A laboratory-developed test which "has not received FDA clearance or approval"; offered to monitor inflammation in asthma and to guide corticosteroid dosing
Labcorp, test 004180	ImmunoCAP	"4−54 μg/L"	Same 60–120 minute standing time before centrifugation
Respiratory Medicine systematic review	Review of published series	Normal serum ECP under 20 µg/L	"serum sampling of ECP should be standardized as far as possible because levels can vary depending on the storage time and ambient temperature"
A paediatric allergy series	Standardised collection, 8 am to 12 pm	Treated children 11.3 µg/L (IQR 7.1–16.1); untreated 35.1 µg/L (IQR 29.5–50.9)	Its conclusion is the protocol again: "To make ECP measurements useful in clinical practices, it is necessary to meet standardized pre-analytical conditions"

Where ECP sits against the biomarkers guidance actually names

This is the honest answer to "what is ECP used for". It is a direct measure of eosinophil activation rather than of eosinophil number, which is a genuinely attractive idea, and it separates treated from untreated asthmatic children in published series. It has nonetheless not found a routine clinical role: the current severe-asthma guidance builds its phenotyping and its biologic-eligibility rules on the eosinophil count, FeNO, sputum eosinophils and IgE, and does not mention ECP. If you are holding an ECP result, convert it, note the protocol, and then read the absolute eosinophil count calculator, which is the measurement the guidance is written around.
Marker	Cut-point in the GINA 2025 severe asthma guide	Status
Blood eosinophil count	"≥150/μl" suggests type 2 inflammation; "≥150 and ≤1500/μl" for anti-IL4Rα eligibility	Named, with a number
FeNO	"≥20 ppb" on initial assessment; "≥25 ppb" for anti-IL4Rα	Named, with a number
Sputum eosinophils	"≥2%"	Named, with a number
Total serum IgE	"Total serum IgE and body weight within local dosing range" for anti-IgE	Named, for dosing
Eosinophil cationic protein	—	Not mentioned anywhere in the document

An analyte that is still being made inside the specimen

The conversion is a formality. A microgram per litre and a nanogram per millilitre are the same concentration written two ways, because the thousandfold in the prefix cancels the thousandfold in the volume, and a picogram per millilitre is a thousandth of that, so pg/mL is the µg/L figure times a thousand. No measured constant appears anywhere, and no molar unit is offered: eosinophil cationic protein is a small glycosylated ribonuclease whose mass varies with its glycosylation and which is measured against an assay calibrator, so it has no single grams per mole to divide by. If relabelling the number is all you came for, it is done above.

What makes ECP unusual enough to deserve a page is that the specimen keeps generating it. Eosinophils store ECP in their secondary granules, and when blood is left to clot the cells go on degranulating into the serum. The measured concentration is therefore a sum of what was circulating in the patient and what the cells released in the tube, and the second term depends on how long the sample stood and how warm it was. Both of the reference laboratories quoted above build that into their instructions rather than treating it as a caveat: clot for 60 to 120 minutes, one of them adding at 20 to 24 °C, then spin at 1000 to 1300 g for ten minutes. A systematic review puts the temperature effect at two to fourfold between 22 and 37 °C over a single hour. That is not a pre-analytical nuisance around the edges of the result. It is larger than the differences the test is being asked to detect.

The same mechanism explains a second oddity. Serum ECP runs five to ten times higher than plasma ECP, because EDTA inactivates the eosinophils and stops the release — so a laboratory that switched matrix would produce numbers that are not on the same scale as its own historical ones. And it explains why the two published adult intervals above are threefold apart. Some of that is method, but some of it is that each interval carries its own laboratory’s protocol inside it. The practical rule that follows is narrow and worth stating plainly: an ECP is interpretable against the interval printed beside it and against previous results from the same laboratory, and against nothing else. If your report does not state the clotting protocol, the number cannot be compared with a reference interval at all — which is the single most useful thing this page can tell you.

As for what the test is for: ECP is a direct index of eosinophil activation rather than of eosinophil number, and that is a real distinction. A patient can have a modest eosinophil count with heavily degranulating cells, and the count will not show it. Published series do separate treated from untreated asthmatic children, and ECP has been studied in asthma monitoring, corticosteroid titration, allergic rhinitis, atopic dermatitis and eosinophilic gastrointestinal disease. It has nonetheless not established a routine clinical role. The current GINA severe-asthma guidance names blood eosinophils, FeNO, sputum eosinophils and total IgE, each with a cut-point, and does not mention ECP; one of the two reference laboratories offering it records that the assay is a laboratory-developed test without FDA clearance. The honest summary is that it is a good idea with a difficult specimen and no agreed threshold.

If you are working up eosinophilia rather than monitoring asthma, the numbers that carry published thresholds are elsewhere: the absolute eosinophil count calculator turns a white cell count and a percentage into the absolute count, the eosinophilia severity interpreter reads that against the figures that define mild, moderate and hypereosinophilia, and the total IgE converter and specific IgE class interpreter cover the allergic side of the same workup.

Frequently asked questions

Is µg/L the same as ng/mL for ECP?

Yes, exactly and always. One microgram in a litre is one nanogram in a millilitre: the prefix changes by a thousand and the volume changes by a thousand, and the two cancel. No factor is involved, which is why laboratories print whichever of the two they prefer without offering a conversion. A picogram per millilitre is a thousandth of a nanogram per millilitre, so pg/mL is the µg/L figure multiplied by 1,000.

Why does the clotting time matter for an ECP result?

Because eosinophils keep releasing ECP into the specimen after it is drawn. The measured concentration is what was circulating plus what the cells degranulated in the tube, and the second part grows with time and with temperature. Two reference laboratories specify clotting for 60 to 120 minutes before centrifugation, one of them at 20 to 24 °C. A systematic review reports that an hour at 37 °C rather than 22 °C raises the result two to fourfold — bigger than most differences anyone would call clinically meaningful.

What is a normal eosinophil cationic protein level?

There is no single answer, which is itself the finding. One reference laboratory gives less than 17.8 µg/L on a fluorescent enzyme immunoassay; another gives 4 to 54 µg/L on ImmunoCAP; a systematic review quotes under 20 µg/L as normal. The upper limits differ threefold on the same assay family, partly through method and partly because each interval has its own pre-analytical protocol built into it. Use the interval printed on your own report.

Can I compare an ECP from one laboratory with an ECP from another?

Not reliably. The result depends on the clotting time and temperature as well as on the assay, and the published adult intervals differ by a factor of three. Serial ECP measurements are interpretable within one laboratory using one protocol; a single result compared against a figure from somewhere else, or against a number in a paper, is not. If the report does not state the protocol, treat the comparison as unavailable rather than approximate.

Is ECP measured in serum or plasma?

Serum, and the two are not interchangeable. Serum ECP is typically five to ten times higher than plasma ECP, because EDTA inactivates eosinophils and prevents the in-tube release that serum deliberately allows. That is why the protocol specifies a clotting period at all: the assay is calibrated against serum handled that way. A plasma result read against a serum interval is out by most of an order of magnitude.

Is ECP a routine test for asthma?

No. It is a direct measure of eosinophil activation rather than of eosinophil number, and published series do separate treated from untreated asthmatic children, but it has not established a routine clinical role. The GINA 2025 severe-asthma guide gives cut-points for blood eosinophils (≥150/µL), FeNO (≥20 ppb), sputum eosinophils (≥2%) and total serum IgE, and does not mention ECP at all. One reference laboratory notes that its assay is a laboratory-developed test without FDA clearance.

Related calculators

References

ARUP Laboratories. Eosinophil Cationic Protein (ECP), test 3000537. "Release ECP by clotting for 60-120 minutes at room temperature (20-24° C)"; "Centrifuge at 1000-1300 x g for 10 minutes at room temperature"; reference interval "Less than 17.8 ug/L"; quantitative fluorescent enzyme immunoassay; laboratory-developed test, not FDA cleared or approved.
Labcorp. Eosinophil Cationic Protein (ECP), test 004180. "Allow the sample to stand for 60 to 120 minutes to release the ECP in the cells prior to centrifugation"; reference interval "4−54 μg/L"; ImmunoCAP.
Koh GC-H, Koh MS. Eosinophil cationic protein: is it useful in asthma? A systematic review. Respir Med. 2007. "ECP secretion in blood samples can also increase by two to four times at 37 °C relative to 22 °C when stored for an hour"; "The levels of ECP in serum is often five to ten times of those in plasma because eosinophils in blood containing EDTA are inactivated".
Blood sample processing effect on eosinophil cationic protein concentration. Ann Allergy Asthma Immunol. 1997. Clotting at 0 °C, room temperature and 37 °C for one hour: "a higher temperature during blood clotting resulted in a higher ECP concentration".
Eosinophil cationic protein in children with respiratory allergies — when is it useful? Lab Med. 2011;42(7):419. Treated children 11.3 µg/L (IQR 7.1–16.1), untreated 35.1 µg/L (IQR 29.5–50.9); "To make ECP measurements useful in clinical practices, it is necessary to meet standardized pre-analytical conditions".
Global Initiative for Asthma. Difficult-to-treat and severe asthma in adolescent and adult patients, 2025. Type 2 biomarker cut-points: blood eosinophils ≥150/µL, FeNO ≥20 ppb, sputum eosinophils ≥2%. ECP is not mentioned.

Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.