How to tell if eggs are bad

Egg floats in water → it is older (larger air cell), not necessarily unsafe
Off smell or pink/iridescent white when cracked → spoiled, discard
Cracked or damaged shell → do not use

Why it happens

FSIS says, in Shell Eggs from Farm to Table, that even unbroken, clean, fresh eggs may contain Salmonella Enteritidis. Eggs have to be handled safely, refrigerated promptly and cooked thoroughly. Spoilage is a separate problem from that hidden risk. FSIS says a pink or iridescent egg white is caused by Pseudomonas bacteria, some of which are harmful to humans.

The float test measures something else. FSIS explains that an egg has no air cell when it is laid. One forms as the egg cools and grows as moisture and carbon dioxide escape through the shell. An egg floats once that air cell is big enough to keep it buoyant. FSIS says this means the egg "is of poor quality, but it may be perfectly safe to use". The FSA's home food fact checker says it does not suggest using the float test to show whether eggs are safe, as "it assesses age, not safety". The test cannot show which bacteria have been growing inside the egg.

Several things look worrying but are not, according to FSIS. A blood spot comes from a small blood vessel rupturing as the egg forms and does not mean the egg is unsafe. A cloudy white is a sign the egg is very fresh, and a clear white means it is ageing. A green ring on a hard-cooked yolk is safe to eat.

What the guidance says

1. Check the shell first. FSIS says bacteria can enter eggs through cracks and tells you never to buy cracked eggs. The FSA's salmonella fact check says not to use eggs with damaged shells, as dirt or bacteria may have got inside.
2. Check the date and how long they have been stored. FSIS and FoodSafety.gov give raw eggs in the shell 3 to 5 weeks in a fridge at 40°F (4°C) or below. Hard-cooked eggs get 1 week, and raw whites or yolks get 2 to 4 days. The FSA says eggs are safe for a couple of days after the best before date if they are cooked well.
3. Crack it into a bowl. FSIS advises cracking a doubtful egg into a bowl and checking it for an off odour or unusual appearance before you decide whether to use it. Throw it away if it smells unpleasant or the white is pink or iridescent.
4. Cook it thoroughly. FSIS says to cook eggs until both the white and yolk are firm, and to cook dishes containing eggs to 160°F (71.1°C). If you use eggs past the best before date, the FSA says to cook both white and yolk thoroughly.
5. Do not wash eggs. FSIS says washing eggs at home increases the risk of contamination, because wash water can be drawn into the egg through the pores in the shell. The FSA says the same: shells are porous, and washing lets bacteria from the outside of the shell into the egg.

Check that it worked

For egg dishes, FSIS gives 160°F (71.1°C) on a food thermometer. For fried, boiled or scrambled eggs, the FSIS check is a firm white and a firm yolk. After cooking, FSIS says to serve eggs straight away, or put them in shallow containers in the fridge and use them within 3 to 4 days. It says hard-cooked eggs should go into the fridge within 2 hours of cooking and be used within a week.

US vs UK guidance on storage and runny eggs

Fridge or cupboard. FSIS says to take eggs straight home and put them in a fridge at 40°F (about 4°C, a conversion) or below. Keep them in their carton in the coldest part of the fridge, not in the door, and do not leave refrigerated eggs out for more than 2 hours. FSIS explains that a cold egg left at room temperature can sweat, which helps bacteria move into the egg. The FSA says to store eggs in a cool, dry place, ideally the fridge. It warns against places with extreme temperature changes, because condensation on the shell lets Salmonella get into the egg more easily. Its chilling guidance also suggests the fridge's egg tray to keep eggs separate from other foods.

Runny yolks. FSIS advice is to cook eggs until white and yolk are firm. The FSA says that eggs with the British Lion mark, or eggs produced under the Laid in Britain scheme, can be eaten raw or lightly cooked. That includes young children, pregnant people, older people and people with some health conditions. The exception is people who are severely immunocompromised. The FSA advice covers UK hen eggs from those schemes only.

Freezing. FSIS and FoodSafety.gov say not to freeze eggs in the shell. They give raw egg whites 12 months in the freezer and say yolks do not freeze well. The FSA says you can crack eggs into a container and freeze them, whole, beaten, or with whites and yolks separated.

If you feel ill after eating eggs, the NHS food poisoning page lists the symptoms and when to speak to a pharmacist, call 111 or call 999. This page gives no medical advice. For cooking temperatures of other foods, see the pork temperature page.

To sum up, a bad smell or a pink or iridescent white means spoilage, and a floating egg is only an old one. An egg that passes both checks can still carry bacteria, so the safety steps are good storage, an intact shell and thorough cooking.