nanogram
Meaning of nanogram: A billionth of a gram, the scale at which hormones and drug levels in the blood are actually measured.
Definition of nanogram
A nanogram is a unit of mass equal to one billionth of a gram. It is written ng and is used where the quantity involved is far too small for everyday units, particularly for hormones, drugs and markers measured in blood. Laboratory results are often reported in nanograms per milliliter, meaning the mass present in each milliliter of the sample.
How small is it?
| Unit | Symbol | Fraction of a gram |
|---|---|---|
| Gram | g | One |
| Milligram | mg | One thousandth |
| Microgram | mcg | One millionth |
| Nanogram | ng | One billionth |
| Picogram | pg | One trillionth |
Each step down that list divides by a thousand. A single grain of table salt weighs a fraction of a milligram, so a nanogram is a tiny fraction of anything that could be seen at all. One gram holds a thousand million of them.
Where does it appear?
Substances active at very low concentrations are reported this way. Thyroid hormone and cortisol, the prostate marker PSA, levels of drugs whose useful range is narrow, and markers of heart muscle injury are all commonly reported in nanograms per milliliter. The unit reflects biological potency rather than importance. A hormone that works in nanogram amounts is not less significant than a mineral measured in milligrams, only more powerful for its weight.
Why do units cause errors?
Two problems recur. Microgram and milligram look alike when handwritten and differ by a factor of a thousand, which is why abbreviating microgram is discouraged in prescribing. And laboratories in different countries report the same substance differently, some using mass per volume such as nanograms per milliliter and others using molar units such as nanomoles per liter. Converting between them requires the molecular weight of the substance, so a number copied from one report into another system without conversion can be badly wrong.
What does per milliliter add?
A mass on its own says how much substance is present. A concentration says how much is present in a given volume, and it is concentration that determines the effect on the body. Nanograms per milliliter and micrograms per liter are numerically identical, since both express the same ratio, and reports may use either. Reference ranges are tied to the unit printed beside them, so a result and its range are always read together.
Also known as
Used in a sentence
The assay reported the hormone concentration in nanograms per milliliter.
Good to know
A result in nanograms per milliliter is a concentration rather than a total amount, and the same substance may be reported in molar units elsewhere, so a number means nothing without the unit printed alongside it.
Word origin
Where “nanogram” comes from
Nano comes from the Greek nanos, meaning dwarf, and marks a billionth part, while gram comes from the Greek gramma, a small unit of weight.