Whether you use in-house analysers or submit samples to the laboratory for testing, it is worth remembering that reference intervals are usually derived from healthy adults, and that significant physiological differences occur between neonates/juveniles and adult dogs.
Haematology:
At birth, haemoglobin (Hb), haematocrit (HCT) and red cell count (RCC) are close to normal adult levels, but decline rapidly over the first 2 months, then increase gradually to reach adult levels by 6 months to 1 year of age, depending on the breed. The drop in red cell mass is significant, as the HCT can be as low as 24%. In neonatal puppies, the MCV can be 95-100 fl, but with the replacement of foetal erythrocytes by adult erythrocytes, this value reaches adult levels by around 3-4 months.
Healthy adult dogs have < 1.0% reticulocytes in peripheral blood, but puppies can have up to 10% reticulocytes in the first 2 months, decreasing to adult levels by approximately 5-6 months of age. A neonatal puppy has a leukocyte count and differential similar to an adult high-normal, however, they have little marrow reserve; with the result that sepsis will create a rapid neutropenia.
Biochemistry:
Glucose levels are important, as puppies, especially toy-breeds, are prone to hypoglycaemia due to fasting or stress. As a general rule, blood glucose of less than 1.7 mmol/L in a neonatal pup, or less than 2.2 mmol/L in a 2-6 week old pup, indicates hypoglycaemia.
Total protein (TP) increases rapidly 12-24 hours after birth, due to colostrum ingestion. Both TP and albumin will then fall below the adult reference interval. In puppies that are less than 8 weeks old, the TP may be around 34-39 g/L, and then between 2-6 months of age, will gradually increase to 50-70 g/L.
Urine SG is significantly lower in 0-3 week old pups, ranges from 1.006-1.017 can be used to estimate hydration status, as a urine SG >1.017 in a neonate would suggest dehydration. Mild glucosuria and proteinuria may be noted for up to 6 weeks.
Creatinine levels in puppies are below adult reference intervals, due to their small muscle mass (particularly toy breeds). Under 12 weeks of age it may be in the range of 26.5-53 umol/L. Serum urea may be at the high end of the adult reference interval for 7 days, and then decline to the low end of the adult reference interval.
Phosphate and calcium are generally higher in dogs less than 1 year old, with larger breeds have higher calcium levels up to around 3 months, and smaller breeds have higher calcium levels up to around 1 week old, with a gradual decline as they mature to adulthood.
ALP and GGT may be extremely high in puppies less than 2 weeks old, due to intestinal absorption of colostrum, but then fall significantly by 4 weeks. ALP activity up to 6-7 months of age is mostly reflective of bone iso-enzyme and may be 2-3 fold higher than adult reference intervals.
