Metabolic models are unlikely to accurately predict microbial crude protein outflow (MCP) from the rumen and amino acid (AA) limitations since they are generally based on static values of unknown accuracy. Practical and reliable methods to identify MCP and AA limitations are required to evaluate nutritional limitations in commercial groups of dairy cows in ‘real time’. Our objective was to determine normal ranges of plasma AA, and MCP, in early lactation multiparity Holstein dairy cows fed a range of rations as a benchmark to determine high, low and normal levels. Multiparity high cow groups on 19 commercial dairies in California were selected. One load of total mixed ration (TMR) was collected at feeding, and milk yields were obtained from monthly milk tests on each dairy. Spot urine samples were collected from voluntarily urinating cows between 38 and 151 d in milk (DIM) to focus on early lactation. Specific gravity of each urine sample was determined to estimate urine flow. Urine samples from each dairy were analyzed for allantoin, which was used, with estimated urine volume, to calculate MCP. Blood was collected from the tail vein of 12 cows/dairy (65 to 76 DIM) and assayed for AA. Group averages among the dairies were 33.6 to 52.5 kg/d milk yield, 1309 to 2149 g/d MCP, 2.8 to 4.4, 8.4 to 13.9, 5.9 to 9.7, 22.9 to 38.5, 16.9 to 27.5 and 11.3 to 18.6 µg/mL for Met, Lys, His, Val, Leu, and Ile respectively. Correlation analysis was performed using the STEPWISE Procedure of SAS with a backward elimination of parameters. Milk and DIM were poor MCP predictors (r² < 0.01 and 0.05 respectively) and neutral detergent fiber (NDF; P = 0.02, r² = 0.23) was the only TMR nutrient that was predictive of MCP, with higher diet NDF levels leading to increased MCP flow. Multiple correlation with all TMR nutrients (% DM) created the best fit model: MCP (g/d) = −5793.1 + (118.8 × NDF) + (150.1 × Fat) + (87.0 × Starch) + (74.0 × nonfiber carbohydrate) [r² = 0.50]. Results show that plasma AA (from 6 cows) and MCP (from 8 cows) are sufficient to determine whether plasma AA and/or MCP values in a group of cows are low, high, or normal thereby suggesting changes in rations to address the identified issue(s).