What fecal lactoferrin is, and whose it is

Fecal lactoferrin is lactoferrin measured in a stool specimen. The molecule is the same iron-binding glycoprotein that occurs in milk and in mucosal secretions, so naming the analyte does not say where it came from, and the test depends entirely on that question. In an adult the answer is neutrophils. In a breast-fed infant it is partly the feed.

The protein that defines a neutrophil granule

Masson, Heremans and Schonne identified lactoferrin inside human neutrophilic leukocytes in 1969, which placed a milk protein in a white blood cell and gave the stool assay its eventual rationale. Where in the cell it sits was settled later, and the answer is specific enough to be used as a laboratory landmark.

Bainton's immuno-electron microscopy work assigns one marker protein to each neutrophil compartment: myeloperoxidase to the azurophilic or primary granule, lactoferrin to the specific or secondary granule, gelatinase to the tertiary granule, albumin to the secretory vesicles. Lactoferrin is the protein by which the specific granule is recognized under the microscope.

A neutrophil that degranulates releases the contents of those compartments. So lactoferrin appearing in the gut lumen records an event rather than a concentration: neutrophils arrived at the bowel wall and emptied their specific granules. Stool concentration rises because that traffic rose. The granule chemistry behind it belongs to the biology of lactoferrin, and the protein itself is the one described in the entry on what lactoferrin is.

What the specimen represents

A stool sample is not a biopsy. It integrates whatever reached the lumen across the passage of that stool, which makes the figure a rate of arrival rather than a reading of the mucosa at any one point. Two things have to hold for the number to mean anything. The protein has to survive the specimen, which is the reason lactoferrin was taken forward instead of three other neutrophil proteins, covered on the lactoferrin test entry. And the protein has to have come from the patient.

The baseline is not zero. Kane and colleagues measured a mean of 1.45 µg/g in 56 healthy adults, against 1,125 µg/g in 80 patients with ulcerative colitis. Some neutrophil traffic into a healthy bowel is normal and the assay detects it.

In a breast-fed infant the lactoferrin is not the infant's

Human milk carries lactoferrin at grams per liter. Healthy adult stool carries it at micrograms per gram. Those two units describe different things and no conversion between them is published, so the gap cannot honestly be given as a ratio. The milk figure is the larger by a wide margin, and enough of it survives an infant's digestion to reach the stool intact. A 1997 cohort in Lima shows what that does to the test.

Huicho and colleagues assessed 125 consecutive children with acute diarrhea for fecal lactoferrin, fecal leukocytes and occult blood, using a rapid agglutination assay read at a 1:50 dilution. Alongside them they tested four groups of 15 control children who had no diarrhea at all. Positivity in those controls sorted itself by feeding method.

Every fully breast-fed child without diarrhea tested positive. Among the children who did have diarrhea, all those on full or mixed breast-feeding tested positive as well, so the assay had no discriminating power left in that group. The authors stated the consequence directly: breast-feeding lowers the specificity of the test and does not alter its sensitivity.

The gradient across those four rows is the useful part. Positivity falls as breast milk falls, which is internal evidence that the milk is the source rather than the bowel. The 20% in exclusively bottle-fed controls shows that breast milk does not account for all of it, and the study does not establish what the remainder was.

A Tulane group published a second cohort on the same question in 1999, in Nicaraguan children, examining intestinal helminthiasis alongside breast-feeding. PubMed carries no abstract for that record, so its figures are not quoted here. It is listed because a reader checking this point should know a second dataset exists.

What a raised value identifies

Neutrophils in the bowel, and nothing narrower. The marker is agnostic about what drew them there, which the infectious disease literature demonstrates better than the inflammatory bowel disease literature does.

Kirkpatrick and colleagues studied three groups of Haitian children under 18 months: children with acute cryptosporidiosis, children whose diarrhea had another cause, and healthy controls. Fecal lactoferrin was significantly raised in the cryptosporidiosis group at P=0.01, alongside interleukin-8, tumor necrosis factor-alpha receptor I, interleukin-13 and interleukin-10. A protozoan parasite moves this number.

In the Lima cohort, lactoferrin showed greater overall sensitivity than fecal leukocytes or occult blood for detecting invasive pathogens, and pairing it with either of the others gave sensitivity and negative predictive value close to 100%. The abstract reports no sensitivity figure for lactoferrin alone, so the comparison is a ranking and the near-100% applies to the combinations rather than to the single test.

Both findings place the same limit on the assay. A raised result says neutrophils are entering the gut, and infection, drug injury and malignancy all recruit them. This is why every pooled accuracy figure on the fecal lactoferrin hub is tied to a named comparison group, and why specificity measured against irritable bowel syndrome promises nothing about an undifferentiated population. Where lactoferrin is being measured in stool, the question of whose lactoferrin it is comes before the question of how much.

This page describes what the assay detects and where the analyte originates. It does not interpret a result. A measured value belongs to the clinician who ordered the test and who knows how the patient is fed.